Specific modulation of sodium channels in mammalian nerve by monoclonal antibodies

Voltage-gated sodium channels

Voltage-gated sodium channels play a key role in the transmission of sensory information about the status of organs in the periphery. Sensory fibers contain a heterogeneous yet specific distribution of voltage-gated sodium channel isoforms. Major efforts by industry and academic groups are underway to develop medicines that interrupt inappropriate signaling for a number of clinical indications by taking advantage of this specific distribution of channel isoforms. This review highlights recent advances in the study of human channelopathies, animal toxins and channel structure that may facilitate the development of selective voltage-gated sodium channel blockers.

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.

Specific antibodies to the external vestibule of voltage-gated potassium channels block current

Using delayed-rectifier potassium channels as examples, we have designed two specific blockers by generating specific antipeptide antibodies to epitopes in the external vestibules of two channel proteins, Kv1.2 and Kv3.1. These antibodies reduced whole-cell Kv1.2 or Kv3.1 currents in transfected cells and the effect was blocked by the corresponding peptide antigen, but not by control peptides. A control antibody had little effect on Kv1.2 currents and the Kv1.2 blocker antibody had limited effect on other related potassium currents. Furthermore, the Kv1.2 blocking antibody inhibited dendrotoxin binding to Kv1.2 channel proteins in transfected cells. Moreover, using the Kv1.2 blocker antibody, we determined the presence and relative contribution of endogenous Kv1.2 to the overall endogenous K+ currents in NG108 neuronal cells. This guided design of specific channel blockers will facilitate future physiological studies on ion channel functions.

Antibodies to GM1 and Gal(beta 1-3)GalNAc at the nodes of Ranvier in human and experimental autoimmune neuropathy

Autoantibodies to Gal(beta 1-3)GalNAc epitopes on glycolipids and glycoproteins are associated with motor neuron disease and motor or sensorimotor neuropathy. These epitopes are ubiquitously distributed on cell surfaces. In the nervous system they are present on axons and myelin, specifically also at the nodes of Ranvier. Binding of GM1 antibodies to the nodal area may contribute to disease development in some of these conditions.

Antibodies as probes for ligand gating of single sarcoplasmic reticulum Ca2(+)-release channels

A large (565 kDa) junctional sarcoplasmic reticulum (SR) protein, the ryanodine receptor (RYR), may play both a structural and a functional role in the mechanism of skeletal muscle excitation-contraction coupling. Recently, the primary amino acid sequence of the RYR has been elucidated. In this paper, we introduce an immunological approach to examine the functional (electrophysiological) properties of the RYR when it is incorporated into planar lipid bilayers. The effects of two polyclonal antibodies against the SR junctional face membrane (JFM) and the RYR (anti-JFM and anti-RYR) were tested on the single-channel gating properties of the RYR SR Ca2(+)-release channel. Junctional SR vesicles were fused into planar lipid bilayers in solutions containing caesium salts. Solutions were designed to minimize the background conductances of the SR K+ and Cl- channels. Three actions of the anti-JFM antibody were distinguished on the basis of single-channel gating and conductance. The anti-RYR antibody had a single action, a simultaneous decrease in single-channel open probability (Po) and conductance. Both antibodies appear to alter single-channel gating by disrupting the Ca2(+)-activation mechanism of the channel. Anti-RYR-antibody-induced gating abnormalities were reversed by ATP, although the ATP-re-activated channels had altered gating kinetics. Two antigenic regions, recognizing the anti-RYR antibody, in the C-terminal end of the RYR primary amino acid sequence contain or are closely associated with putative ligand (Ca2+ and ATP)-binding sites identified previously. Our results demonstrate (1) that the antibodies induced abnormal gating (decreased open probability and stabilization of subconducting states) of SR release channels, and (2) that abnormal gating is not associated with physical obstruction or alteration of the conduction pathway. Thus antibodies directed at specific regions of the RYR (e.g. putative ligand-binding sites) can be used as effective probes with which to study the structural and functional properties of the SR Ca2(+)-release channel gating at the single-channel level.

Experimental autoimmune neuropathy with anti-GM1 antibodies and immunoglobulin deposits at the nodes of Ranvier

Antibodies to GM1 or Gal(beta 1-3)GalNAc are associated with motor or sensorimotor neuropathy and with motor neuron disease. To investigate the role of these antibodies in the neurological disorder, rabbits were immunized with GM1 or with Gal(beta 1-3)GalNAc-BSA, and studied serologically, electrophysiologically and pathologically. Development of antibodies to the immunizing antigens was associated with a fall in the ratio of the amplitudes of the compound muscle action potential evoked by proximal versus distal stimulation of the sciatic nerve. Pathological studies revealed mild axonal degeneration and immunoglobulin deposits at the nodes of Ranvier in peripheral nerve, resembling those reported in a patient with motor neuropathy, motor conduction block and anti-GM1 antibodies. These studies provide evidence that anti-GM1 or anti-Gal(beta 1-3)GalNAc antibodies cause conduction abnormalities and indicate that the antibodies may exert their effect, in part, by binding at the nodes of Ranvier in peripheral nerve.

IgM deposits at nodes of Ranvier in a patient with amyotrophic lateral sclerosis, anti-GM1 antibodies, and multifocal motor conduction block

We studied a patient with amyotrophic lateral sclerosis, multifocal motor conduction block, and IgM anti-GM1 antibodies. A sural nerve biopsy demonstrated deposits of IgM at nodes of Ranvier by direct immunofluorescence. The deposits were granular and located in the nodal gap between adjacent myelin internodes, and in some instances, they extended along the surface of the paranodal myelin sheath. When injected into rat sciatic nerve, the serum IgM bound to the nodes of Ranvier, and the binding activity was removed by preincubation with GM1. These observations suggest that anti-GM1 antibodies may have caused motor dysfunction by binding to the nodal and paranodal regions of peripheral nerve.

Antibodies to junctional sarcoplasmic reticulum proteins: probes for the Ca2+-release channel

The junctional face membrane plays a key role in excitation-contraction coupling in skeletal muscle. A protein of 350 kDa, tentatively identified as a component of the junctional feet, connects transverse tubules to terminal cisternae of sarcoplasmic reticulum [Kawamoto, Brunschwig, Kim & Caswell (1986) J. Cell Biol. 103, 1405-1414]. The membrane topology and protein composition of sarcoplasmic reticulum Ca2+-release channels of rabbit skeletal muscle were investigated using an immunological approach, with anti-(junctional face membrane) and anti-(350 kDa protein) polyclonal antibodies. Upon preincubation of the terminal cisternae with anti-(junctional face membrane) antibodies, Ca2+-ATPase and Ca2+-loading activities were not affected, whereas anti-(350 kDa protein) antibodies stimulated Ca2+-ATPase activity by 25% and inhibited Ca2+-loading activity by 50% (at an antibody/terminal cisternae protein ratio of 1:1). Specific photolabelling of terminal cisternae proteins with [14C]doxorubicin was prevented by both anti-(junctional face membrane) and anti-(350 kDa protein) antibodies. Stimulation of Ca2+ release by doxorubicin was prevented by both anti-(junctional face membrane) and anti-(350 kDa protein) antibodies. Half-maximal inhibition was obtained at an antibody/terminal cisternae protein ratio of 1:1. Kinetic measurements of Ca2+ release indicated that anti-(350 kDa protein) antibodies prevented Ca2+-induced Ca2+ release, whereas the ATP-stimulation and the inhibition by Mg2+ were not affected. These results suggest that: (i) Ca2+- and doxorubicin-induced Ca2+ release is mediated by Ca2+ channels which are selectively localized in the junctional face membrane; (ii) the 350 kDa protein is a component of the Ca2+-release channel in native terminal cisternae vesicles; and (iii) the Ca2+-activating site of the channel is separate from other allosteric sites.

Mapping a region associated with Na channel inactivation using antibodies to a synthetic peptide corresponding to a part of the channel

Antibodies to the synthetic peptide (carrier-coupled) corresponding to amino acids 210-223 of the primary sequence of eel Na channel (C1+ peptide) were generated. The antipeptide antibodies were used to identify functional roles as well as the accessibility from the external membrane surface of the C1+ domains. Rabbit antipeptide antibodies bound specifically to the C1+ synthetic peptide and to an eel membrane fraction bearing a high density of Na channels. When applied to the external surface of cultured dorsal root ganglion cells obtained from newborn rats, the antibodies modify Na channel inactivation by shifting the steady-state Na current-inactivation parameter, h infinity, curve to more negative potentials in fast and slow Na currents. The rate of inactivation of the slow channel is shown to be increased. The antibodies do not have a significant effect on activation of the channels. Part of the amino acid sequence corresponding to C1+ peptide is therefore accessible, in the mammalian Na channel, from the external membrane surface and is associated with the inactivation gate.

Detection of sodium channel distribution in rat sciatic nerve following lysophosphatidylcholine-induced demyelination

In vivo application of lysophosphatidylcholine (LPC) to rat sciatic nerve induces impaired hind leg movement within 2 days which is recovered by 6 days. Segmental demyelination was seen at 2 days after LPC application, and remyelination had barely started in a few axons by 6 days. Using sodium channel-specific monoclonal antibodies and immunofluorescence microscopy, we observed altered distribution of sodium channels in demyelinated axons. Bright fluorescent labeling was found along the segmentally demyelinated axolemma at 6 days in contrast to the dim staining of the demyelinated nerve found at 2 days. In addition, radioimmunoassays detected an elevated number of antibody binding sites on sciatic nerve trunk from the sixth day. Our data provide the immunocytochemical evidence for the assumption that recruitment of sodium channels into demyelinated axolemma contributes to the recovery of function following axon demyelination by LPC.

Monoclonal antibodies associated with sodium channel block nerve impulse and stain nodes of Ranvier

Monoclonal antibodies were generated against native eel electroplax sodium channels in their natural membrane. These antibodies block nerve conduction in rat central (optic) and peripheral (sciatic) nerve. The antibody binding to eel electroplax membrane fragments and to rat brain synaptosomes can be modulated by neurotoxins. Thus it implies that active sites of the sodium channels are immunogenic in their natural membrane. Unlike the antibodies described in the past, our antibodies recognize antigenic determinants which are associated with the physiological activity of the channel and have been conserved through evolution.