Gating of Ion Channels

Excitable cells, such as neurons and muscles, use ion channels to generate electrical and chemical signals that underlie their functions. Examples include the electrical signals underlying the complex neuronal circuitry in the brain, the secretion of hormones and neurotransmitters, skeletal and cardiac muscle contraction, and the signaling events that lead to fertility. Because of their pivotal role in cellular signaling and electrical excitability, a major goal in modern biology has been to determine the physical properties that control and modulate ion channel function. This chapter briefly reviews classical works about the gating of ion channels. Furthermore, it discusses some innovative approaches that when combined with biophysical and mathematical simulations have contributed to the current understanding of channel gating.

Acid-Sensing Ion Channels

Acid-sensing ion channels (ASICs) are proton-gated Na+ channels. Being almost ubiquitously present in neurons of the vertebrate nervous system, their precise function remained obscure for a long time. Various animal toxins that bind to ASICs with high affinity and specificity have been tremendously helpful in uncovering the role of ASICs. We now know that they contribute to synaptic transmission at excitatory synapses as well as to sensing metabolic acidosis and nociception. Moreover, detailed characterization of mouse models uncovered an unanticipated role of ASICs in disorders of the nervous system like stroke, multiple sclerosis, and pathological pain. This review provides an overview on the expression, structure, and pharmacology of ASICs plus a summary of what is known and what is still unknown about their physiological functions and their roles in diseases.

AMPA and Kainate Receptors

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate-type glutamate receptors (AMPARs and KARs) are dynamic ion channel proteins that govern neuronal excitation and signal transduction in the mammalian brain. The four AMPAR and five KAR subunits can heteromerize with other subfamily members to create several combinations of tetrameric channels with unique physiological and pharmacological properties. While both receptor classes are noted for their rapid, millisecond-scale channel gating in response to agonist binding, the intricate structural rearrangements underlying their function have only recently been elucidated. This chapter begins with a review of AMPAR and KAR nomenclature, topology, and rules of assembly. Subsequently, receptor gating properties are outlined for both single-channel and synaptic contexts. The structural biology of AMPAR and KAR proteins is also discussed at length, with particular focus on the ligand-binding domain, where allosteric regulation and alternative splicing work together to dictate gating behavior. Toward the end of the chapter there is an overview of several classes of auxiliary subunits, notably transmembrane AMPAR regulatory proteins and Neto proteins, which enhance native AMPAR and KAR expression and channel gating, respectively. Whether bringing an ion channel novice up to speed with glutamate receptor theory and terminology or providing a refresher for more seasoned biophysicists, there is much to appreciate in this summation of work from the glutamate receptor field.

Potassium Channel Mutations in Epilepsy

This chapter describes recent advances in understanding the clinical significance of rare variants in potassium channel genes in the causation of epilepsy. The α subunits of potassium channels fall into three major families, which are encoded by at least 70 different genes, of which at least 40 are brain-expressed. Brain-expressed potassium channels, in both nerve and glial cells, have complex roles in the regulation of neurodevelopment and cortical excitability. The chapter discusses the 20 potassium channel α-subunit genes in which rare variants have been linked to a wide variety of neurocognitive phenotypes. Advances in the understanding of how gene variants affect channel function to result in neuronal dysfunction and epilepsy are discussed, as well as descriptions of the phenotypic characteristics of the disorder and how a genetic diagnosis currently impacts clinical management. The rapid discovery of potassium channelopathies causal of epilepsy needs to be matched by improved understanding of the impact of individual variants within the human brain in order to develop truly targeted therapies that will result in seizure control and potentially improved neurodevelopmental outcome.

Excitable Membrane Properties of Neurons

The intrinsic electrical properties of neurons are extremely varied. For example, the width of action potentials in different neurons varies by more than an order of magnitude. In response to prolonged stimulation, some neurons generate repeated action potential hundreds of times a second, while others fire only a single action potential or adapt very rapidly. These differences result from the expression of different types of ion channels in the plasma membrane. The dominant channels that shape neuronal firing patterns are those that are selective for sodium, calcium, and potassium ions. This chapter provides a brief overview of the biophysical properties of each of these classes of channel, their role in shaping the electrical personality of a neuron, and how interactions of these channels with cytoplasmic factors shape the overall cell biology of a neuron.

N-Methyl-D-Aspartate Receptors

Discovered more than 70 years ago due to advances in electrophysiology and cell culture techniques, N-methyl-D-aspartate (NMDA) receptors remain the target of assiduous basic and clinical research. This interest flows from their intimate engagement with fundamental processes in the mammalian central nervous system and the resulting natural desire to understand how this receptor’s genetically encoded structural properties generate their distinctive functional features and how in turn these unique functional attributes play into the larger opus of physiological and pathological processes. From the overwhelming literature on the subject, the authors briefly outline contemporary understanding of the receptor’s evolutionary origins, molecular diversity, and expression patterns; sketch hypothesized correlations between structural dynamics, signal kinetics, and pathophysiological consequences; and highlight the breadth of processes in which NMDA receptors are implicated, many of which remain poorly understood. Continued developments in cryo-electron microscopy, whole-genome sequencing and editing, imaging, and other emerging technologies will likely confirm some of the current hypotheses and challenge others to produce a more accurate reflection of these receptors’ complex operation and myriad roles in health and disease.

Ion Channel Permeation and Selectivity

Many biological processes essential for life rely on the transport of specific ions at specific times across cell membranes. Such exquisite control of ionic currents, which is regulated by protein ion channels, is fundamental for the proper functioning of the cells. It is not surprising, therefore, that the mechanism of ion permeation and selectivity in ion channels has been extensively investigated by means of experimental and theoretical approaches. These studies have provided great mechanistic insight but have also raised new questions that are still unresolved. This chapter first summarizes the main techniques that have provided significant knowledge about ion permeation and selectivity. It then discusses the physical mechanisms leading to ion permeation and the explanations that have been proposed for ion selectivity in voltage-gated potassium, sodium, and calcium channels.

The Voltage-Dependent K+ Channel Family

Voltage-dependent K+ (potassium; Kv) channels are ion channels that critically impact neuronal excitability and function. Four principal α subunits assemble to create a membrane-spanning pore that opens in a voltage-dependent manner to allow the selective passage of K+ ions across the cell membrane. Forty human genes encoding Kv channel α subunits have been identified, and most of them are expressed in the nervous system. The individual Kv subunits display unique cellular and subcellular expression patterns and co-assemble into distinct homo- and hetero-tetrameric channels that differ in their electrophysiological and pharmacological properties, and their sensitivity to dynamic modulation, by cellular signaling pathways. The resulting diversity allows Kv channels to impact all steps in electrical information processing, as well as numerous other aspects of neuronal functions, including those in which they appear to play a non-conducting role. This chapter reviews the current basic knowledge about this large and important family of ion channels.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are members of the voltage-gated K+ channels family, but with unique properties. In stark contrast to close relatives, HCN channels are permeable to both Na+ and K+, and they are activated by hyperpolarization. Activation by hyperpolarization is indeed a pretty funny feature, to the point that the physiologists who first characterized HCN current in heart muscle cells named it “funny current” or I f. Since then, the funny current has also been recorded from several neuronal types in both the central and peripheral nervous systems, as well as from some non-excitable cells, becoming progressively less “funny” over the years. In fact, HCN current goes now by the more serious designation of “I h,” for “hyperpolarization-activated.” Forty years after the first current recording, it is now established that HCN channels, by virtue of their special properties and a host of modulatory mechanisms, are profoundly involved in many critical aspects of neuronal function in physiological and pathological conditions.

Tandem Pore Domain Potassium Channels

The KCNK gene family encodes two-pore-domain potassium (K2P) channels, which generate the background (“leak”) K+ currents that establish a negative resting membrane potential in cells of the nervous system. A pseudotetrameric K+-selective pore is formed by pairing channel subunits, each with two pore-domains, in homo- or heterodimeric conformations. Unique features apparent from high-resolution K2P channel structures include a domain-swapped extracellular cap domain, a lateral hydrophobic-lined fenestration connecting the lipid bilayer to the channel vestibule, and an antiparallel proximal C-terminal region that links the paired subunits and provides a site for polymodal channel modulation. Individual channels transition between open and closed states, with the channel gate located at the selectivity filter. In general, K2P channels display relatively modest voltage- and time-dependent gating, together with distinct single-channel rectification properties, that conspire to yield characteristic weakly rectifying macroscopic currents over a broad range of membrane potentials (i.e., background K+ currents). Of particular note, K2P channel activity can be regulated by a wide range of physicochemical factors, neuromodulators, and clinically useful drugs; a distinct repertoire of activators and inhibitors for different K2P channel subtypes endows each with unique modulatory potential. Thus, by mediating background currents and serving as targets for multiple modulators, K2P channels are able to dynamically regulate key determinants of cell-intrinsic electroresponsive properties. The roles of specific K2P channels in various physiological processes and pathological conditions are now beginning to come into focus, and this may portend utility for these channels as potential therapeutic targets.