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Ion Channels: Structural Basis for Function and Disease UC Irvine UC Irvine Previously Published Works Title Ion channels: structural basis for function and disease. Permalink https://escholarship.org/uc/item/39x307jx Journal Seminars in perinatology, 20(6) ISSN 0146-0005 Author Goldstein, SA Publication Date 1996-12-01 DOI 10.1016/s0146-0005(96)80066-8 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Ion Channels: Structural Basis for Function and Disease Steve A. N. Goldstein Ion channels are ubiquitous proteins that mediate nervous and muscular function, rapid transmem- brane signaling events, and ionic and fluid balance. The cloning of genes encoding ion channels has led to major strides in understanding the mechanistic basis for their function. These advances have shed light on the role of ion channels in normal physiology, clarified the molecular basis for an expanding number of diseases, and offered new direction to the development of rational therapeutic interventions. Copyright 1996 by W.B. Saunders Company on channels reside in the membranes of all by ion channels to be divided into two broad cells and control their electrical activity. 1 mechanistic groups: those resulting from loss of These proteins underlie subtle biological events channel function and those consequent to gain such as the response of a single rod cell to a of channel function. Three exemplary patho- beam of light, the activation of a T cell by its physiological correlates are examined, Long QT antigen, and the fast block to polyspermy of a syndrome, Liddle's syndrome and pseudohypo- fertilized ovum. They also mediate spectacular aldosteronism type 1 (leading to life-threatening events like heart beats, systemic fluid and electro- cardiac rhythm disturbances, systemic hyperten- lyte homeostasis, and our memories. Over the sion, and hypotension, respectively). Future past 40 years, increasingly sensitive electrophysi- challenges for ion channel research are consid- ological methods have allowed characterization ered. of channel function at remarkable resolution- single channel molecules can be observed in real Ion Channel Function: Gating and Ion time (Fig 1A). However, it is only in the last few Selectivity years with the application of molecular genetic Ion channels perform two operations. First, they technology that we have begun to discern the open and close in response to specific stimuli; structural basis for ion channel function. Clon- this process is called "channel gating." Second, ing of genes for channels and their regulatory they catalyze the flux of specific ions across the subunits has revealed amino acid sequences and membrane, an activity referred to as "ion selec- made feasible studies of structure and function tive conduction." Some channels are ligand- that were impossible with native tissues alone. gated. Thus, binding of cGMP to a cyclic-nucleo- Recent advances in our understanding of the tide gated (CNG) channel in the retina leads to mechanistic underpinnings for normal function a conformational change in channel structure of sodium (Na +) and potassium (K+) channels and a shift from a closed, nonconducting chan- are discussed in this article. These insights have nel state to an open state that is maintained as clarified the etiology for an expanding number long as cGMP is bound. Other ion channels are of disease states and allow disorders mediated voltage-gated. The single Na + channel shown in Fig 1A undergoes a series of state transitions From the Section of Pediatric Cardiology, Department of Pediatrics and the Department of Cellular and Molecular Physiology, Yale when the membrane is depolarized from its rest- University School of Medicine, New Haven, CT. ing level (-80 mV) to a more positive potential Supported by grants from the National Institutes of Health and the (-10 mV). The channel moves first from a Donaghue Foundation. closed (or resting) state to an open state that Address reprint requests to Steve A. N. Goldstein, MD, PhD, Boyer allows Na + to flow into the cell; it proceeds to Centerfor Molecular Medicine, Program of Molecular Cardiobiology, Yale University, 295 Congress Ave, New Haven, CT 06536. another nonconducting conformation, the inac- Copyright 1996 by W.B. Saunders Company tive state, in which it sits silently despite main- 0146-0005/96/2006-0006505.00/0 tained membrane depolarization (Fig 1B). 520 Seminars in Perinatology, Vol 20, No 6 (December), 1996: pp 520-530 Ion Channels: Basis for Function and Disease 521 A An open ion channel forms a water-filled con- duction pore across the membrane bringing the closed ~ external and intracellular solutions into continu- open ity. Ions passively diffuse through the pore in a direction determined solely by differences in ion concentration and electrical potential across the membrane. This does not indicate that channel proteins are inert scaffolding; the number and type of ions that move through a channel reveal its influence. Thus, some K + channels catalyze the flow of 100,000,000 K + ions each second through a single channel complex, yet maintain -10 mV a preference for K + over Na § of 10,000 to 1.2 We -80 mV I will consider the dramatic progress in under- standing of the molecular basis for opening and B inactivation of voltage-gated channels as well as the channel regions that mediate ion conduc- Resting .,9[,m=,~ Open ~ Inactivated tion and selectivity. C Ion Channel Function: The Cardiac Action Potential Membrane ~,1 2 Voltage ....... ) #~JIIpsiJl~11sis~# A cell's electrical behavior is determined by the gating and selectivity attributes of its ion chan- nels. :~-5 In the heart, cells of the conduction path- Current way exhibit a pattern of cyclical excitability with ( ) five phases. A Purkinje cell action potential and .4 .... j I.... "" its two primary channel currents are schematized Potassium / in Fig 1C. Phase 0 is an explosive rise in mem- Current ( ) brane potential due to a rush of Na+into the cell (down its concentration gradient) through voltage-gated Na + channels that open with mem- brane depolarization; these channels then rap- idly inactivate (Fig 1A, B). Phase 1 is a brief repo- larization step mediated by voltage-gated K + Figure 1. Single channel recording ofa Na + channel with a gating scheme and a diagram of a cardiac action channels that open with depolarization and, like potential with two contributing channel currents. (A) phase 0 Na + channels, rapidly inactivate. Be- Single channel recording from a cardiac voltage-gated cause outward flow of K + (down its concentra- Na + channels (courtesy of Ted Cummins and Fred tion gradient) makes the cell interior more nega- Sigworth, Yale University). In response to a change in tive, opening K § channels shifts the cell toward voltage from -80 mV to -10 mV, the channel moves from resting to open conformation and then to an more negative potentials. Phase 2, the plateau inactive state. (B) Scheme for gating of an inactivating in the action potential, is coincident with myo- voltage-gated channel. (C) Diagram of the phases of cardial contraction and results from the cumula- a cardiac Purkinje cell action potential with currents tive activity of a number of channel and carrier- through two contributing channels superimposed, type transporters. The duration of phase 2 is based on Fozzard. 6~ The channel currents associated with the five phases of the action potential are: phase determined by voltage-gated K + channels that 0, rapid depolarization-IN, and I(:,; phase 1, fast repo- open with a delay in response to membrane de- larization-It,,; phase 2, plateau-It,; phase 3, delayed re- polarization and remain open until the mem- polarization-I~,, I~r; phase 4, pacemaker depolariza- brane is again hyperpolarized (Fig 1C). These tion-Ira, If. delayed outward K + currents return the mem- brane to its resting potential during phase 3 and 522 Steve A. N. Goldstein allow the heart to relax. Phase 4, the pacemaker nels and epithelial Na + channels share a pro- potential, is a slow rise in membrane potential posed two transmembrane topology (Fig 2C), attributed to closing of other K + channels and whereas the recently identified outward rectifier instigates the next cycle of excitation and con- (Fig 2D) and open rectifier K + channel subunits traction. In broad outline, cardiac excitation re- (Fig 2E) are built with two domains similar to sults from rapid, voltage-dependent gating of those observed in previously identified K + chan- Na + channels, delayed, voltage-dependent gat- nels. ing of K + channels, and the ability of both chan- nels to discriminate between Na + and K § ions. Assigning Structure to Function: Voltage-Dependent Gating Cloning of Genes Encoding Ion Channels Channel regions that participate in gating and Identification of the structural determinants of ion selectivity were first suggested by comparison gating and selectivity followed cloning of the of the amino acid sequences predicted from genes encoding ion channels. The first Na § each newly isolated gene. 6avas Candidate regions channel gene was isolated by a direct biochemi- were then evaluated and influential residues cal approach; channel protein was purified from identified by studying the functional effects of the electric organ of the Electrophorus electricus eel site-directed mutations. in sufficient quantity to allow production of anti- channel antibodies, and these were subsequently Resting to Open Transition used to screen and isolate the gene from an eel cDNA expression
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