Depolarization exposes the voltage sensor of the sodium channels to the extracellular region

Ion channels as therapeutic antibody targets

It is now well established that antibodies have numerous potential benefits when developed as therapeutics. Here, we evaluate the technical challenges of raising antibodies to membrane-spanning proteins together with enabling technologies that may facilitate the discovery of antibody therapeutics to ion channels. Additionally, we discuss the potential targeting opportunities in the anti-ion channel antibody landscape, along with a number of case studies where functional antibodies that target ion channels have been reported. Antibodies currently in development and progressing towards the clinic are highlighted.

Antibody therapeutics targeting ion channels: are we there yet?

The combination of technological advances, genomic sequences and market success is catalyzing rapid development of antibody-based therapeutics. Cell surface receptors and ion channel proteins are well known drug targets, but the latter has seen less success. The availability of crystal structures, better understanding of gating biophysics and validation of physiological roles now form an excellent foundation to pursue antibody-based therapeutics targeting ion channels to treat a variety of diseases.

Immunopharmacology--antibodies for specific modulation of proteins involved in neuronal function

The application of antibodies to living neurones has the potential to modulate function of specific proteins by virtue of their high specificity. This specificity has proven effective in determining the involvement of many proteins in neuronal function where specific agonists and antagonists do not exist, e.g. ion channel subunits. We discuss studies where antibodies modulate functions of voltage gated sodium, voltage gated potassium, voltage gated calcium hyperpolarisation activated cyclic nucleotide (HCN gated) and transient receptor potential (TRP) channels. Ligand gated channels studied in this way include nicotinic acetylcholine receptors, purinoceptors and GABA receptors. Antibodies have also helped reveal the involvement of different intracellular proteins in neuronal functions including G-proteins as well as other proteins involved in trafficking, phosphoinositide signalling and neurotransmitter release. Some suggestions for control experiments are made to help validate the method. We conclude that antibodies can be extremely valuable in determining the functions of specific proteins in living neurones in neuroscience research.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

The screw-helical voltage gating of ion channels

In the voltage-gated ion channels of every animal, whether they are selective for K+, Na+ or Ca2+, the voltage sensors are the S4 transmembrane segments carrying four to eight positive charges always separated by two uncharged residues. It is proposed that they move across the membrane in a screw-helical fashion in a series of three or more steps that each transfer a single electronic charge. The unit steps are stabilized by ion pairing between the mobile positive charges and fixed negative charges, of which there are invariably two located near the inner ends of segments S2 and S3 and a third near the outer end of either S2 or S3. Opening of the channel involves three such steps in each domain.

Electrophorus electricus as a model system for the study of membrane excitability

The stunning sensations produced by electric fish, particularly the electric eel, Electrophorus electricus, have fascinated scientists for centuries. Within the last 50 years, however, electric cells of Electrophorus have provided a unique model system that is both specialized and appropriate for the study of excitable cell membrane electrophysiology and biochemistry. Electric tissue generates whole animal electrical discharges by means of membrane potentials that are remarkably similar to those of mammalian neurons, myocytes and secretory cells. Electrocytes express ion channels, ATPases and signal transduction proteins common to these other excitable cells. Action potentials of electrocytes represent the specialized end function of electric tissue whereas other excitable cells use membrane potential changes to trigger sophisticated cellular processes, such as myofilament cross-bridging for contraction, or exocytosis for secretion. Because electric tissue lacks these functions and the proteins associated with them, it provides a highly specialized membrane model system. This review examines the basic mechanisms involved in the generation of the electrical discharge of the electric eel and the membrane proteins involved. The valuable contributions that electric tissue continues to make toward the understanding of excitable cell physiology and biochemistry are summarized, particularly those studies using electrocytes as a model system for the study of the regulation of membrane excitability by second messengers and signal transduction pathways.

The kinetics of voltage-gated ion channels

When Hodgkin & Huxley (1952) first embarked on the analysis of their voltageclamp data on the ionic currents in the squid giant axon, they hoped to be able to deduce a mechanism from it, but it soon became clear that the electrical data would by themselves yield only very general information about the class of system likely to be involved.

The modulatory site for the action of gadolinium on surface charges and channel gating

The gadolinium (Gd3+)-induced shift of potential dependence and modulated gating of Na and K channels were analyzed. In a previous investigation, we explained the shift in terms of pure screening (no binding) of fixed surface charges and the modulation by binding to modulatory sites on the channels. In the present paper, we have extended this model by including effects on the charge density of Gd3+ binding to the modulatory sites. From fitting the extended model to experimental data, the charge density was estimated to be -0.6 e nm-2, and the Gd(3+)-induced charge change to be +0.15 e nm-2, and the maximal scaling factor to be 7.5 for both Na and K channels. Intrinsic KD values for binding to the K and Na channels were estimated to be 140 and 380 mM, respectively. Estimations of the extracellular charge density, from primary structures of cloned channels, were found to be in agreement with estimations based on the present model. The modulatory site was suggested to be located at the cluster of negatively charged residues between the fifth transmembrane segment (S5) and the pore-forming region for both Na and K channels. These suggestions imply several testable predictions about different K channels.

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles in cardiac ventricular myocytes. Effects via modification of surface potential

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles was investigated in cardiac ventricular myocytes using the patch-clamp technique. Negatively charged sodium dodecylsulfate (SDS) increased amplitude of INa, whereas positively charged dodecyltrimethylammonium (DDTMA) decreased INa. Furthermore, SDS shifted the steady-state activation and inactivation of INa in the negative direction, whereas DDTMA shifted the curves in the opposite direction. These shifts provided an explanation for the changes in current amplitude. Activation and inactivation kinetics of INa were accelerated by SDS but slowed by DDTMA. These changes in both steady-state gating and kinetics of INa are consistent with a decrease of the intramembrane field by SDS and an increase of the field by DDTMA due to an alteration of surface potential after their insertion into the outer monolayer of the sarcolemma. The effect of SDS on the steady-state inactivation of INa was concentration dependent and partially reversed by screening surface charges with increased extracellular [Ca2+]. These amphiphiles also altered the activation of the delayed rectifier K+ current (IK,del), producing a shift in the negative direction by SDS but in the positive direction by DDTMA. These results suggest that the insertion of charged amphiphiles into the cell membrane alters the behavior of voltage-dependent INa and IK,del by changing the surface charge density, and consequently the surface potential and implies, although indirectly, that the lipid surface charges are important to the voltage-dependent gating of these channels.