KCNQ2 is a nodal K+ channel

Molecular microdomains in a sensory terminal, the vestibular calyx ending

Many primary vestibular afferents form large cup-shaped postsynaptic terminals (calyces) that envelope the basolateral surfaces of type I hair cells. The calyceal terminals both respond to glutamate released from ribbon synapses in the type I cells and initiate spikes that propagate to the afferent's central terminals in the brainstem. The combination of synaptic and spike initiation functions in these unique sensory endings distinguishes them from the axonal nodes of central neurons and peripheral nerves, such as the sciatic nerve, which have provided most of our information about nodal specializations. We show that rat vestibular calyces express an unusual mix of voltage-gated Na and K channels and scaffolding, cell adhesion, and extracellular matrix proteins, which may hold the ion channels in place. Protein expression patterns form several microdomains within the calyx membrane: a synaptic domain facing the hair cell, the heminode abutting the first myelinated internode, and one or two intermediate domains. Differences in the expression and localization of proteins between afferent types and zones may contribute to known variations in afferent physiology.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Accumulation of Kv7.2 channels in putative ectopic transduction zones of mice nerve-end neuromas

Background

Modulation of M-type currents has been proposed as a new strategy for the treatment of neuropathic pain due to their role in regulating neuronal excitability. Using electrophysiological techniques we showed previously that the opening of Kv7 channels with retigabine, blocked ectopic discharges from axotomized fibers but did not alter transduction at intact skin afferents. We hypothesized that after nerve damage, accumulation of Kv7 channels in afferent fibers may increase M-type currents which then acquired a more important role at regulating fiber excitability.

Findings

In this study, we used an immunohistochemical approach to examine patterns of expression of Kv7.2 channels in afferent fibers after axotomy and compared them to patterns of expression of voltage gated Na⁺ channels (Nav) which are key electrogenic elements in peripheral axons known to accumulate in experimental and human neuromas.

Axotomy induced an enlargement and narrowing of the nodes of Ranvier at the proximal end of the neuroma together with a dramatic demyelination and loss of structure at its distal end in which naked accumulations of Nav were present. In addition, axotomy also induced accumulations of Kv7.2 that co-localized with those of Nav channels.

Conclusions

Whilst Nav channels are mandatory for initiation of action potentials, (i.e. responsible for the generation/propagation of ectopic discharges) an increased accumulation of Kv7.2 channels after axotomy may represent a homeostatic compensation to over excitability in axotomized fibers, opening a window for a peripheral action of M-current modulators under conditions of neuropathy.

Axons and myelinating glia: An intimate contact

The coordination of the vertebrate nervous system requires high velocity signal transmission between different brain areas. High speed nerve conduction is achieved in the myelinated fibers of both the central and the peripheral nervous system where the myelin sheath acts as an insulator of the axon. The interactions between the glial cell and the adjacent axon, namely axo-glial interactions, segregate the fiber in distinct molecular and functional domains that ensure the rapid propagation of action potentials. These domains are the node of Ranvier, the paranode, the juxtaparanode and the internode and are characterized by multiprotein complexes between voltage-gated ion channels, cell adhesion molecules, members of the Neurexin family and cytoskeletal proteins. In the present review, we outline recent evidence on the key players of axo-glial interactions, depicting their importance in myelinated fiber physiology and disease.

Distribution of M-channel subunits KCNQ2 and KCNQ3 in rat hippocampus

Neuronal M-channels are low threshold, slowly activating and non-inactivating, voltage dependent K(+) channels that play a crucial role in controlling neuronal excitability. The native M-channel is composed of heteromeric or homomeric assemblies of subunits belonging to the Kv7/KCNQ family, with KCNQ2/3 heteromers being the most abundant form. KCNQ2 and KCNQ3 subunits have been found to be expressed in various neurons in the central and peripheral nervous system of rodents and humans. Previous evidence shows preferential localization of both subunits to axon initial segments, somata and nodes of Ranvier. In this work, we show the distribution and co-localization of KCNQ2 and KCNQ3 subunits throughout the hippocampal formation, via immunostaining experiments on unfixed rat brain slices and confocal microscopy. We find intense localization and colocalization to the axonal initial segment in several regions of the hippocampus, as well as staining for non-neuronal cells in the area of the lateral ventricle. We did not observe colocalization of KCNQ2 or KCNQ3 with the presynaptic protein, synaptophysin.

The pharmacological effect of positive KCNQ (Kv7) modulators on dopamine release from striatal slices

Retigabine is an anti-epileptic drug that inhibits neuronal firing by stabilizing the membrane potential through positive modulation of voltage-dependent KCNQ potassium channels in cortical neurons and in mesencephalic dopamine (DA) neurons. The purpose of this study was to compare the effect of retigabine with other positive KCNQ modulators on the KCl-induced release of DA in rat striatal slices. Retigabine was found to inhibit KCl-dependent release of DA, and the IC(50) was estimated to be 0.7 μM. The KCNQ channel blocker XE-991 enhanced striatal DA release and completely abolished the effect of retigabine. Other compounds of the same class but with some preferences for different KCNQ subtypes such as ICA-27243, BMS-204352 and S-(1) were also tested. All three compounds produced a significant effect albeit weaker than retigabine. The potency of ICA-27243 was in the range of retigabine, and with a lower potency of BMS-204352 and S-(1). This study demonstrates that KCNQ channel openers inhibit KCl-induced DA release at relevant concentrations. The equal potency of ICA-27243 and retigabine suggests that the KCNQ2/3 isoform is likely the dominant subtype mediating this effect.

Contributions of Kv7-mediated potassium current to sub- and suprathreshold responses of rat layer II/III neocortical pyramidal neurons

After block of Kv1- and Kv2-mediated K(+) currents in acutely dissociated neocortical pyramidal neurons from layers II/III of rat somatosensory and motor cortex, the remaining current is slowly activating and persistent. We used whole cell voltage clamp to show that the Kv7 blockers linopirdine and XE-991 blocked a current with similar kinetics to the current remaining after combined block of Kv1 and Kv2 channels. This current was sensitive to low doses of linopirdine and activated more slowly and at more negative potentials than Kv1- or Kv2-mediated current. The Kv7-mediated current decreased in amplitude with time in whole cell recordings, but in most cells the current was stable for several minutes. Current in response to a traditional M-current protocol was blocked by muscarine, linopirdine, and XE-991. Whole cell slice recordings revealed that the Q₁₀ for channel deactivation was ∼2.5. Sharp electrode current-clamp recordings from adult pyramidal cells demonstrated that block of Kv7-mediated current with XE-991 reduced rheobase, shortened the latency to firing to near rheobase current, induced more regular firing at low current intensity, and increased the rate of firing to a given current injection. XE-991 did not affect single action potentials or spike frequency adaptation. Application of XE-991 also eliminated subthreshold voltage oscillations and increased gain for low-frequency inputs (<10 Hz) without affecting gain for higher frequency inputs. These data suggest important roles for Kv7 channels in subthreshold regulation of excitability, generation of theta-frequency subthreshold oscillations, regulation of interspike intervals, and biasing selectivity toward higher frequency inputs.

KCNQ5 channels control resting properties and release probability of a synapse

Little is known about which ion channels determine the resting electrical properties of presynaptic membranes. In recordings made from the rat calyx of Held, a giant mammalian terminal, we found resting potential to be controlled by KCNQ (Kv7) K(+) channels, most probably KCNQ5 (Kv7.5) homomers. Unlike most KCNQ channels, which are activated only by depolarizing stimuli, the presynaptic channels began to activate just below the resting potential. As a result, blockers and activators of KCNQ5 depolarized or hyperpolarized nerve terminals, respectively, markedly altering resting conductance. Moreover, the background conductance set by KCNQ5 channels, together with Na(+) and hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, determined the size and time course of the response to subthreshold stimuli. Signaling pathways known to directly affect exocytic machinery also regulated KCNQ5 channels, and increase or decrease of KCNQ5 channel activity controlled release probability through alterations in resting potential. Thus, ion channel determinants of presynaptic resting potential also control synaptic strength.

Function and mechanism of axonal targeting of voltage-sensitive potassium channels

Precise localization of various ion channels into proper subcellular compartments is crucial for neuronal excitability and synaptic transmission. Axonal K(+) channels that are activated by depolarization of the membrane potential participate in the repolarizing phase of the action potential, and hence regulate action potential firing patterns, which encode output signals. Moreover, some of these channels can directly control neurotransmitter release at axonal terminals by constraining local membrane excitability and limiting Ca(2+) influx. K(+) channels differ not only in biophysical and pharmacological properties, but in expression and subcellular distribution as well. Importantly, proper targeting of channel proteins is a prerequisite for electrical and chemical functions of axons. In this review, we first highlight recent studies that demonstrate different roles of axonal K(+) channels in the local regulation of axonal excitability. Next, we focus on research progress in identifying axonal targeting motifs and machinery of several different types of K(+) channels present in axons. Regulation of K(+) channel targeting and activity may underlie a novel form of neuronal plasticity. This research field can contribute to generating novel therapeutic strategies through manipulating neuronal excitability in treating neurological diseases, such as multiple sclerosis, neuropathic pain, and Alzheimer's disease.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Ovarian hormone deficiency reduces intrinsic excitability and abolishes acute estrogen sensitivity in hippocampal CA1 pyramidal neurons

Premature and uncompensated loss of ovarian hormones following ovariectomy (OVX) elevates the risks of cognitive impairment and dementia. These risks are prevented with estrogen (E(2))-containing hormone replacement therapy initiated shortly following OVX but not after substantial delay. Currently, the cellular bases underlying these clinical findings are unknown. At the cellular level, intrinsic membrane properties regulate the efficiency of synaptic inputs to initiate output action potentials (APs), thereby affecting neuronal communication, hence cognitive processing. This study tested the hypothesis that in CA1 pyramidal neurons, intrinsic membrane properties and their acute regulation by E(2) require ovarian hormones for maintenance. Whole-cell current-clamp recordings were performed on neurons from ∼ 7-month-old OVX rats that experienced either short-term (10 d, control OVX) or long-term (5 months, OVX(LT)) ovarian hormone deficiency. The results reveal that long-term hormone deficiency reduced intrinsic membrane excitability (IE) as measured by the number of evoked APs and firing duration for a given current injection. This was accompanied by AP broadening, an increased slow afterhyperpolarization (sAHP), and faster accumulation of Na(V) channel inactivation during repetitive firing. In the control OVX neurons, E(2) acutely increased IE and reduced the sAHP. In contrast, acute regulation of IE by E(2) was absent in the OVX(LT) neurons. Since the degree of IE of hippocampal pyramidal neurons is positively related with hippocampus-dependent learning ability, and modulation of IE is observed following successful learning, these findings provide a framework for understanding hormone deficiency-related cognitive impairment and the critical window for therapy initiation.

Made for "anchorin": Kv7.2/7.3 (KCNQ2/KCNQ3) channels and the modulation of neuronal excitability in vertebrate axons

Kv7.2 and Kv7.3 (encoded by KCNQ2 and KCNQ3) are homologous subunits forming a widely expressed neuronal voltage-gated K(+) (Kv) channel. Hypomorphic mutations in either KCNQ2 or KCNQ3 cause a highly penetrant, though transient, human phenotype-epilepsy during the first months of life. Some KCNQ2 mutations also cause involuntary muscle rippling, or myokymia, which is indicative of motoneuron axon hyperexcitability. Kv7.2 and Kv7.3 are concentrated at axonal initial segments (AISs), and at nodes of Ranvier in the central and peripheral nervous system. Kv7.2 and Kv7.3 share a novel ∼80 residue C-terminal domain bearing an "anchor" motif, which interacts with ankyrin-G and is required for channel AIS (and likely, nodal) localization. This domain includes the sequence IAEGES/TDTD, which is analogous (not homologous) to the ankyrin-G interaction motif of voltage-gated Na(+) (Na(V)) channels. The KCNQ subfamily is evolutionarily ancient, with two genes (KCNQ1 and KCNQ5) persisting as orthologues in extant bilaterian animals from worm to man. However, KCNQ2 and KCNQ3 arose much more recently, in the interval between the divergence of extant jawless and jawed vertebrates. This is precisely the interval during which myelin and saltatory conduction evolved. The natural selection for KCNQ2 and KCNQ3 appears to hinge on these subunits' unique ability to be coordinately localized with Na(V) channels by ankyrin-G, and the resulting enhancement in the reliability of neuronal excitability.

Nodes of ranvier and paranodes in chronic acquired neuropathies

Chronic acquired neuropathies of unknown origin are classified as chronic inflammatory demyelinating polyneuropathies (CIDP) and chronic idiopathic axonal polyneuropathies (CIAP). The diagnosis can be very difficult, although it has important therapeutic implications since CIDP can be improved by immunomodulating treatment. The aim of this study was to examine the possible abnormalities of nodal and paranodal regions in these two types of neuropathies. Longitudinal sections of superficial peroneal nerves were obtained from biopsy material from 12 patients with CIDP and 10 patients with CIAP and studied by immunofluorescence and in some cases electron microscopy. Electron microscopy revealed multiple alterations in the nodal and paranodal regions which predominated in Schwann cells in CIDP and in axons in CIAP. In CIDP paranodin/Caspr immunofluorescence was more widespread than in control nerves, extending along the axon in internodes where it appeared intense. Nodal channels Nav and KCNQ2 were less altered but were also detected in the internodes. In CIAP paranodes, paranodin labeling was irregular and/or decreased. To test the consequences of acquired primary Schwann cells alteration on axonal proteins, we used a mouse model based on induced deletion of the transcription factor Krox-20 gene. In the demyelinated sciatic nerves of these mice we observed alterations similar to those found in CIDP by immunofluorescence, and immunoblotting demonstrated increased levels of paranodin. Finally we examined whether the alterations in paranodin immunoreactivity could have a diagnosis value. In a sample of 16 biopsies, the study of paranodin immunofluorescence by blind evaluators led to correct diagnosis in 70 ± 4% of the cases. This study characterizes for the first time the abnormalities of nodes of Ranvier in CIAP and CIDP, and the altered expression and distribution of nodal and paranodal proteins. Marked differences were observed between CIDP and CIAP and the alterations in paranodin immunofluorescence may be an interesting tool for their differential diagnosis.

Presynaptic action potential waveform determines cortical synaptic latency

Synaptic latency at cortical synapses is determined by the presynaptic release probability (Pr). Short- and long-term presynaptic plasticity is associated with modulation of synaptic delay. We show here that the duration and amplitude of the presynaptic action potential also determine synaptic latency at neocortical and hippocampal excitatory synapses. Blockade of voltage-gated potassium (Kv) channels with 4-aminopyridine or dendrotoxin-I, but not tetraethylammonium, induced a 1–2 ms shift in latency at excitatory synaptic connections formed by pairs of neocortical pyramidal neurons. 4-Aminopyridine or dendrotoxin-I, but not tetraethylammonium, increased the duration of the action potential recorded in the axon, suggesting that presynaptic spike duration is controlled by axonal Kv1 potassium channels. Spike width-dependent changes in latency have been identified at the mossy fibre–CA3 cell synapses and contribute to stabilization of synaptic timing during repetitive stimulation. The effects of presynaptic spike amplitude on synaptic latency were also examined. Decreasing the amplitude of the presynaptic action potential with 15–30 nm TTX reduced synaptic latency by ∼0.5 ms. The regulation of synaptic timing by potassium and sodium channel blockers could not be attributed to modulation of axonal conduction. Rather, these effects are compatible with modifications of the kinetics of the presynaptic calcium current. We conclude that synaptic latency at cortical neurons is not constant but dynamically regulated by presynaptic action potential waveform.

The C-terminal domain of ßIV-spectrin is crucial for KCNQ2 aggregation and excitability at nodes of Ranvier

The spectrin cytoskeleton has an important function in the targeting of proteins to excitable membrane domains. In axons, βIV-spectrin stabilizes voltage-gated sodium (Nav) channel clusters at nodes of Ranvier and axon initial segments, two regions crucial for the generation and conduction of action potentials. Here, I investigated the physiology of the neuromuscular junction and peripheral nerves in quivering-3J mice, which show a frame-shift base insertion in the Spnb4 gene and lack the C-terminus of βIV-spectrin. The quivering-3J mice show prominent spontaneous and evoked hyperactivities at diaphragm neuromuscular junctions. These neuromyotonic and myokymic discharges were more prominent in adult animals when tremors and ataxia were pronounced. Recordings of sciatic and phrenic nerves showed that the hyperactivities originate in myelinated axons distally from nerve terminals. Axon and myelin structure in the PNS were unaffected in quivering-3J mice. Of interest, KCNQ2 subunit aggregates were undetectable at PNS and CNS nodes, whereas Nav and Kv1.1/Kv1.2 channels were properly concentrated at nodal and juxtaparanodal regions, respectively. The protein level of KCNQ2 subunits was normal in mutant animals, suggesting that KCNQ2 subunit absence stems from clustering or trafficking defects in axons. The quivering-3J nodes also presented high densities of ankyrin-G and CK2α, two cytosolic molecules involved with aggregating Nav and KCNQ2/3 channels in axons. Because βIV-spectrin does not interact with KCNQ2/3 subunits, it is suspected that βIV-spectrin regulates the distribution of KCNQ2/3 subunits in axonal subdomains via regulatory partners. Retigabine, an activator of KCNQ2/3 channels, attenuated the repetitive activities in quivering-3J mice, suggesting that depletion of KCNQ2 subunits at nodes initiates neuromyotonic/myokymic discharges. These findings demonstrate that spectrin cytoskeleton finely regulates ion channel distribution and implicates KCNQ2/3 subunits in axonal excitability and in myokymia aetiology.

Composition, assembly, and maintenance of excitable membrane domains in myelinated axons

Neurons have many specialized membrane domains with diverse functions responsible for receiving, integrating, and transmitting electrical signals between cells in a circuit. Both the locations and protein compositions of these domains defines their functions. In axons, two of the most important membrane domains are the axon initial segment and the nodes of Ranvier. Proper assembly and maintenance of these domains is necessary for action potential generation and propagation, and the overall function of the neuron.

Determinants of voltage-gated sodium channel clustering in neurons

In mammalian neurons, the generation and propagation of the action potential result from the presence of dense clusters of voltage-gated sodium channels (Nav) at the axonal initial segment (AIS) and nodes of Ranvier. In these two structures, the assembly of specific supra-molecular complexes composed of numerous partners, such as cytoskeletal scaffold proteins and signaling proteins ensures the high concentration of Nav channels. Understanding how neurons regulate the expression and discrete localization of Nav channels is critical to understanding the diversity of normal neuronal function as well as neuronal dysfunction caused by defects in these processes. Here, we review the mechanisms establishing the clustering of Nav channels at the AIS and in the node and discuss how the alterations of Nav channel clustering can lead to certain pathophysiologies.

P/Q and N channels control baseline and spike-triggered calcium levels in neocortical axons and synaptic boutons

Cortical axons contain a diverse range of voltage-activated ion channels, including Ca(2+) currents. Interestingly, Ca(2+) channels are not only located at presynaptic terminals, but also in the axon initial segment (AIS), suggesting a potentially important role in the regulation of action potential generation and neuronal excitability. Here, using two-photon microscopy and whole-cell patch-clamp recording, we examined the properties and role of calcium channels located in the AIS and presynaptic terminals of ferret layer 5 prefrontal cortical pyramidal cells in vitro. Subthreshold depolarization of the soma resulted in an increase in baseline and spike-triggered calcium concentration in both the AIS and nearby synaptic terminals. The increase in baseline calcium concentration rose with depolarization and fell with hyperpolarization with a time constant of approximately 1 s and was blocked by removal of Ca(2+) from the bathing medium. The increases in calcium concentration at the AIS evoked by subthreshold or suprathreshold depolarization of the soma were blocked by the P/Q-channel antagonist omega-agatoxin IVA or the N-channel antagonist omega-conotoxin GVIA or both. The presence of these channels in the AIS pyramidal cells was confirmed with immunochemistry. Block of these channels slowed axonal action potential repolarization, apparently from reduction of the activation of a Ca(2+)-activated K(+) current, and increased neuronal excitability. These results demonstrate novel mechanisms by which calcium currents may control the electrophysiological properties of axonal spike generation and neurotransmitter release in the neocortex.

Clustered K+ channel complexes in axons

Voltage-gated K+ (Kv) channels regulate diverse neuronal properties including action potential threshold, amplitude, and duration, frequency of firing, neurotransmitter release, and resting membrane potential. In axons, Kv channels are clustered at a variety of functionally important sites including axon initial segments, juxtaparanodes of myelinated axons, nodes of Ranvier, and cerebellar basket cell terminals. These channels are part of larger protein complexes that include cell adhesion molecules and scaffolding proteins. These interacting proteins play important roles in recruiting K+ channels to distinct axonal domains. Here, I review the composition, functions, and mechanism of localization of these K+ channel complexes in axons.

Threshold predictions of different pulse shapes using a human auditory nerve fibre model containing persistent sodium and slow potassium currents

The ability of a human auditory nerve fibre computational model to predict threshold differences for biphasic, pseudomonophasic and alternating monophasic waveforms was investigated. The effect of increasing the interphase gap, interpulse interval and pulse rate on thresholds was also simulated. Simulations were performed for both anodic-first and cathodic-first stimuli. Results indicated that the model correctly predicted threshold reductions for pseudomonophasic compared to biphasic waveforms, although reduction for alternating monophasic waveforms was underestimated. Threshold reductions were more pronounced for cathodic-first stimuli compared to anodic-first stimuli. Reversal of the phases in pseudomonophasic stimuli suggested a threshold reduction for anodic-first stimuli, but a threshold increase in cathodic-first stimuli. Inclusion of the persistent sodium and slow potassium currents in the model resulted in a reasonably accurate prediction of the non-monotonic threshold behaviour for pulse rates higher than 1000 pps. However, the model did not correctly predict the threshold changes observed for low pulse rate biphasic and alternating monophasic waveforms. It was suggested that these results could in part be explained by the difference in the refractory periods between real and simulated auditory nerve fibres, but also by the lack of representation of stochasticity observed in real auditory nerve fibres in our auditory nerve model.

The Kv7 potassium channel activator flupirtine affects clinical excitability parameters of myelinated axons in isolated rat sural nerve

Flupirtine is an activator of Kv7 (KCNQ/M) potassium channels that has found clinical use as an analgesic with muscle relaxant properties. Kv7 potassium channels are expressed in axonal membranes and pharmacological activation of these channels may restore abnormal nerve excitability. We have examined the effect of flupirtine on the electrical excitability of myelinated axons in isolated segments of rat sural nerve. Axonal excitability was studied in vitro with the same parameters used by clinical neurophysiologists to assess peripheral nerve excitability in situ. Application of flupirtine in low micromolar concentrations resulted in an increase in threshold current, a reduction of refractoriness and an increase in post-spike superexcitability. These effects are consistent with an increase in Kv7 conductance and membrane hyperpolarization. Flupirtine also enhanced and prolonged the late, long-lasting period of axonal subexcitability that follows a short burst of action potentials. This effect was blocked by XE 991 (10 microM), an antagonist of Kv7 channels. In summary, flupirtine affects measures of excitability that are altered in the myelinated axons of patients with peripheral nerve disorders. This indicates that neuropathies with abnormal nerve excitability parameters corresponding to those affected by flupirtine may benefit from activation of axonal Kv7 potassium channels.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Axon initial segment dysfunction in a mouse model of genetic epilepsy with febrile seizures plus

Febrile seizures are a common childhood seizure disorder and a defining feature of genetic epilepsy with febrile seizures plus (GEFS+), a syndrome frequently associated with Na+ channel mutations. Here, we describe the creation of a knockin mouse heterozygous for the C121W mutation of the beta1 Na+ channel accessory subunit seen in patients with GEFS+. Heterozygous mice with increased core temperature displayed behavioral arrest and were more susceptible to thermal challenge than wild-type mice. Wild-type beta1 was most concentrated in the membrane of axon initial segments (AIS) of pyramidal neurons, while the beta1(C121W) mutant subunit was excluded from AIS membranes. In addition, AIS function, an indicator of neuronal excitability, was substantially enhanced in hippocampal pyramidal neurons of the heterozygous mouse specifically at higher temperatures. Computational modeling predicted that this enhanced excitability was caused by hyperpolarized voltage activation of AIS Na+ channels. This heat-sensitive increased neuronal excitability presumably contributed to the heightened thermal seizure susceptibility and epileptiform discharges seen in patients and mice with beta1(C121W) subunits. We therefore conclude that Na+ channel beta1 subunits modulate AIS excitability and that epilepsy can arise if this modulation is impaired.

Persistent currents and discharge patterns in rat hindlimb motoneurons

We report here the first direct measurements of persistent inward currents (PICs) in rat hindlimb motoneurons, obtained from ketamine-xylazine anesthetized rats during slow voltage ramps performed by single-electrode somatic voltage clamp. Most motoneurons expressed PICs and current-voltage (I-V) relations often contained a negative-slope region (NSR; 13/19 cells). PICs activated at -52.7 ± 3.89 mV, 9 mV negative to spike threshold. NSR onset was -44.2 ± 4.1 mV. PIC amplitudes were assessed by maximum inward currents measured relative to extrapolated leak current and to NSR-onset current. PIC conductance at potentials just positive to activation was assessed by the relative change in slope conductance (g(in)/g(leak)). PIC amplitudes varied widely; some exceeded 5 and 10 nA relative to current at NSR onset or leak current, respectively. PIC amplitudes did not vary significantly with input conductance, but PIC amplitudes normalized by recruitment current decreased with increasing input conductance. Similarly, g(in)/g(leak) decreased with increasing input conductance. Currents near resting potential on descending limbs of I-V relations were often outward, relative to ascending-limb currents. This residual outward current was correlated with increases in leak conductance on the descending limb and with input conductance. Excluding responses with accommodation, residual outward currents matched differences between recruitment and derecruitment currents, suggesting a role for residual outward current in frequency adaptation. Comparison of potentials for PIC activation and NSR onset with interspike trajectories during discharge demonstrated correspondence between PIC activation and frequency-current (f-I) range boundaries. Contributions of persistent inward and outward currents to motoneuron discharge characteristics are discussed.

Bral1: its role in diffusion barrier formation and conduction velocity in the CNS

At the nodes of Ranvier, excitable axon membranes are exposed directly to the extracellular fluid. Cations are accumulated and depleted in the local extracellular nodal region during action potential propagation, but the impact of the extranodal micromilieu on signal propagation still remains unclear. Brain-specific hyaluronan-binding link protein, Bral1, colocalizes and forms complexes with negatively charged extracellular matrix (ECM) proteins, such as versican V2 and brevican, at the nodes of Ranvier in the myelinated white matter. The link protein family, including Bral1, appears to be the linchpin of these hyaluronan-bound ECM complexes. Here we report that the hyaluronan-associated ECM no longer shows a nodal pattern and that CNS nerve conduction is markedly decreased in Bral1-deficient mice even though there were no differences between wild-type and mutant mice in the clustering or transition of ion channels at the nodes or in the tissue morphology around the nodes of Ranvier. However, changes in the extracellular space diffusion parameters, measured by the real-time iontophoretic method and diffusion-weighted magnetic resonance imaging (MRI), suggest a reduction in the diffusion hindrances in the white matter of mutant mice. These findings provide a better understanding of the mechanisms underlying the accumulation of cations due to diffusion barriers around the nodes during saltatory conduction, which further implies the importance of the Bral1-based extramilieu for neuronal conductivity.

KV7 channelopathies

KV7 voltage-gated potassium channels, encoded by the KCNQ gene family, have caught increasing interest of the scientific community for their important physiological roles, which are emphasized by the fact that four of the five so far identified members are related to different hereditary diseases. Furthermore, these channels prove to be attractive pharmacological targets for treating diseases characterized by membrane hyperexcitability. KV7 channels are expressed in brain, heart, thyroid gland, pancreas, inner ear, muscle, stomach, and intestines. They give rise to functionally important potassium currents, reduction of which results in pathologies such as long QT syndrome, diabetes, neonatal epilepsy, neuromyotonia, or progressive deafness. Here, we summarize some key traits of KV7 channels and review how their molecular deficiencies could explain diverse disease phenotypes. We also assess the therapeutic potential of KV7 channels; in particular, how the activation of KV7 channels by the compounds retigabine and R-L3 may be useful for treatment of epilepsy or cardiac arrhythmia.

Electrogenic tuning of the axon initial segment

Action potentials (APs) provide the primary means of rapid information transfer in the nervous system. Where exactly these signals are initiated in neurons has been a basic question in neurobiology and the subject of extensive study. Converging lines of evidence indicate that APs are initiated in a discrete and highly specialized portion of the axon-the axon initial segment (AIS). The authors review key aspects of the organization and function of the AIS and focus on recent work that has provided important insights into its electrical signaling properties. In addition to its main role in AP initiation, the new findings suggest that the AIS is also a site of complex AP modulation by specific types of ion channels localized to this axonal domain.

A rapamycin-sensitive signaling pathway is essential for the full expression of persistent pain states

Translational control through the mammalian target of rapamycin (mTOR) is critical for synaptic plasticity, cell growth, and axon guidance. Recently, it was also shown that mTOR signaling was essential for the maintenance of the sensitivity of subsets of adult sensory neurons. Here, we show that persistent pain states, but not acute pain behavior, are substantially alleviated by centrally administered rapamycin, an inhibitor of the mTOR pathway. We demonstrate that rapamycin modulates nociception by acting on subsets of primary afferents and superficial dorsal horn neurons to reduce both primary afferent sensitivity and central plasticity. We found that the active form of mTOR is present in a subpopulation of myelinated dorsal root axons, but rarely in unmyelinated C-fibers, and heavily expressed in the dorsal horn by lamina I/III projection neurons that are known to mediate the induction and maintenance of pain states. Intrathecal injections of rapamycin inhibited the activation of downstream targets of mTOR in dorsal horn and dorsal roots and reduced the thermal sensitivity of A-fibers. Moreover, in vitro studies showed that rapamycin increased the electrical activation threshold of Adelta-fibers in dorsal roots. Together, our results imply that central rapamycin reduces neuropathic pain by acting both on an mTOR-positive subset of A-nociceptors and lamina I projection neurons and suggest a new pharmacological route for therapeutic intervention in persistent pain states.

Complementary theta resonance filtering by two spatially segregated mechanisms in CA1 hippocampal pyramidal neurons

Synaptic input to a neuron may undergo various filtering steps, both locally and during transmission to the soma. Using simultaneous whole-cell recordings from soma and apical dendrites from rat CA1 hippocampal pyramidal cells, and biophysically detailed modeling, we found two complementary resonance (bandpass) filters of subthreshold voltage signals. Both filters favor signals in the theta (3-12 Hz) frequency range, but have opposite location, direction, and voltage dependencies: (1) dendritic H-resonance, caused by h/HCN-channels, filters signals propagating from soma to dendrite when the membrane potential is close to rest; and (2) somatic M-resonance, caused by M/Kv7/KCNQ and persistent Na(+) (NaP) channels, filters signals propagating from dendrite to soma when the membrane potential approaches spike threshold. Hippocampal pyramidal cells participate in theta network oscillations during behavior, and we suggest that that these dual, polarized theta resonance mechanisms may convey voltage-dependent tuning of theta-mediated neural coding in the entorhinal/hippocampal system during locomotion, spatial navigation, memory, and sleep.

Kv7/KCNQ channels control action potential phasing of pyramidal neurons during hippocampal gamma oscillations in vitro

While the synaptic mechanisms involved in the generation of in vitro network oscillations have been widely studied, little is known about the importance of voltage-gated currents during such activity. Here we study the role of the M-current (I(M)) in the modulation of network oscillations in the gamma-frequency range (20-80 Hz). Kv7/KCNQ subunits, the molecular correlates of I(M), are abundantly expressed in CA1 and CA3 pyramidal neurons, and I(M) is an important modulator of pyramidal neuron firing. Using hippocampal slices, we recorded field activity and pyramidal neuron action potential timing during kainate-induced gamma oscillations. Application of the specific I(M) blocker XE991 causes a significant reduction of gamma oscillation amplitude with no significant change in oscillation frequency. Concomitant CA3 pyramidal neuron recordings show a significant increase in action potential frequency during ongoing gamma oscillations after the application of XE991. This increase is associated with a significant loss of periodicity of pyramidal neuron action potentials relative to the phase of the gamma oscillations. Using dynamic clamp, we show that I(M) acts to improve the periodicity of action potential timing and to decrease action potential frequency. We further validate these results in a compartmental model of a pyramidal neuron. Our work suggests that I(M) modulates gamma oscillations by regulating the phasing of action potential firing in pyramidal neurons.

KV7/M channels mediate osmotic modulation of intrinsic neuronal excitability

Modest decreases in extracellular osmolarity induce brain hyperexcitability that may culminate in epileptic seizures. At the cellular level, moderate hyposmolarity markedly potentiates the intrinsic neuronal excitability of principal cortical neurons without significantly affecting their volume. The most conspicuous cellular effect of hyposmolarity is converting regular firing neurons to burst-firing mode. This effect is underlain by hyposmotic facilitation of the spike afterdepolarization (ADP), but its ionic mechanism is unknown. Because blockers of K(V)7 (KCNQ) channels underlying neuronal M-type K(+) currents (K(V)7/M channels) also cause spike ADP facilitation and bursting, we hypothesized that lowering osmolarity inhibits these channels. Using current- and voltage-clamp recordings in CA1 pyramidal cells in situ, we have confirmed this hypothesis. Furthermore, we show that hyposmotic inhibition of K(V)7/M channels is mediated by an increase in intracellular Ca(2+) concentration via release from internal stores but not via influx of extracellular Ca(2+). Finally, we show that interfering with internal Ca(2+)-mediated inhibition of K(V)7/M channels entirely protects against hyposmotic ADP facilitation and bursting, indicating the exclusivity of this novel mechanism in producing intrinsic neuronal hyperexcitability in hyposmotic conditions.

Membrane domains based on ankyrin and spectrin associated with cell-cell interactions

Nodes of Ranvier and axon initial segments of myelinated nerves, sites of cell-cell contact in early embryos and epithelial cells, and neuromuscular junctions of skeletal muscle all perform physiological functions that depend on clustering of functionally related but structurally diverse ion transporters and cell adhesion molecules within microdomains of the plasma membrane. These specialized cell surface domains appeared at different times in metazoan evolution, involve a variety of cell types, and are populated by distinct membrane-spanning proteins. Nevertheless, recent work has shown that these domains all share on their cytoplasmic surfaces a membrane skeleton comprised of members of the ankyrin and spectrin families. This review will summarize basic features of ankyrins and spectrins, and will discuss emerging evidence that these proteins are key players in a conserved mechanism responsible for assembly and maintenance of physiologically important domains on the surfaces of diverse cells.

Selective interaction of syntaxin 1A with KCNQ2: possible implications for specific modulation of presynaptic activity

KCNQ2/KCNQ3 channels are the molecular correlates of the neuronal M-channels, which play a major role in the control of neuronal excitability. Notably, they differ from homomeric KCNQ2 channels in their distribution pattern within neurons, with unique expression of KCNQ2 in axons and nerve terminals. Here, combined reciprocal coimmunoprecipitation and two-electrode voltage clamp analyses in Xenopus oocytes revealed a strong association of syntaxin 1A, a major component of the exocytotic SNARE complex, with KCNQ2 homomeric channels resulting in a ∼2-fold reduction in macroscopic conductance and ∼2-fold slower activation kinetics. Remarkably, the interaction of KCNQ2/Q3 heteromeric channels with syntaxin 1A was significantly weaker and KCNQ3 homomeric channels were practically resistant to syntaxin 1A. Analysis of different KCNQ2 and KCNQ3 chimeras and deletion mutants combined with in-vitro binding analysis pinpointed a crucial C-terminal syntaxin 1A-association domain in KCNQ2. Pull-down and coimmunoprecipitation analyses in hippocampal and cortical synaptosomes demonstrated a physical interaction of brain KCNQ2 with syntaxin 1A, and confocal immunofluorescence microscopy showed high colocalization of KCNQ2 and syntaxin 1A at presynaptic varicosities. The selective interaction of syntaxin 1A with KCNQ2, combined with a numerical simulation of syntaxin 1A's impact in a firing-neuron model, suggest that syntaxin 1A's interaction is targeted at regulating KCNQ2 channels to fine-tune presynaptic transmitter release, without interfering with the function of KCNQ2/3 channels in neuronal firing frequency adaptation.

Enhancing m currents: a way out for neuropathic pain?

Almost three decades ago, the M current was identified and characterized in frog sympathetic neurons (Brown and Adams, 1980). The years following this discovery have seen a huge progress in the understanding of the function and the pharmacology of this current as well as on the structure of the underlying ion channels. Therapies for a number of syndromes involving abnormal levels of excitability in neurons are benefiting from research on M currents. At present, the potential of M current openers as analgesics for neuropathic pain is under discussion. Here we offer a critical view of existing data on the involvement of M currents in pain processing. We believe that enhancement of M currents at the site of injury may become a powerful strategy to alleviate pain in some peripheral neuropathies.

Modelled temperature-dependent excitability behaviour of a generalised human peripheral sensory nerve fibre

The objective of this study was to determine if a recently developed human Ranvier node model, which is based on a modified version of the Hodgkin-Huxley model, could predict the excitability behaviour in human peripheral sensory nerve fibres with diameters ranging from 5.0 to 15.0 microm. The Ranvier node model was extended to include a persistent sodium current and was incorporated into a generalised single cable nerve fibre model. Parameter temperature dependence was included. All calculations were performed in Matlab. Sensory nerve fibre excitability behaviour characteristics predicted by the new nerve fibre model at different temperatures and fibre diameters compared well with measured data. Absolute refractory periods deviated from measured data, while relative refractory periods were similar to measured data. Conduction velocities showed both fibre diameter and temperature dependence and were underestimated in fibres thinner than 12.5 microm. Calculated strength-duration time constants ranged from 128.5 to 183.0 micros at 37 degrees C over the studied nerve fibre diameter range, with chronaxie times about 30% shorter than strength-duration time constants. Chronaxie times exhibited temperature dependence, with values overestimated by a factor 5 at temperatures lower than body temperature. Possible explanations include the deviated absolute refractory period trend and inclusion of a nodal strangulation relationship.

A KCNQ channel opener for experimental neonatal seizures and status epilepticus

OBJECTIVE

Neonatal seizures occur frequently, are often refractory to anticonvulsants, and are associated with considerable morbidity and mortality. Genetic and electrophysiological evidence indicates that KCNQ voltage-gated potassium channels are critical regulators of neonatal brain excitability. This study tests the hypothesis that selective openers of KCNQ channels may be effective for treatment of neonatal seizures.

METHODS

We induced seizures in postnatal day 10 rats with either kainic acid or flurothyl. We measured seizure activity using quantified behavioral rating and electrocorticography. We compared the efficacy of flupirtine, a selective KCNQ channel opener, with phenobarbital and diazepam, two drugs in current use for neonatal seizures.

RESULTS

Unlike phenobarbital or diazepam, flupirtine prevented animals from experiencing development of status epilepticus when administered before kainate. In the flurothyl model, phenobarbital and diazepam increased latency to seizure onset, but flupirtine completely prevented seizures throughout the experiment. Flupirtine was also effective in arresting electrographic and behavioral seizures when administered after animals had developed continuous kainate-induced status epilepticus. Flupirtine caused dose-related sedation and suppressed electroencephalographic activity but did not result in respiratory suppression or result in any mortality.

INTERPRETATION

Flupirtine appears more effective than either of two commonly used antiepileptic drugs, phenobarbital and diazepam, in preventing and suppressing seizures in both the kainic acid and flurothyl models of symptomatic neonatal seizures. KCNQ channel openers merit further study as potential treatments for seizures in infants and children.

Neural KCNQ (Kv7) channels

KCNQ genes encode five Kv7 K(+) channel subunits (Kv7.1-Kv7.5). Four of these (Kv7.2-Kv7.5) are expressed in the nervous system. Kv7.2 and Kv7.3 are the principal molecular components of the slow voltage-gated M-channel, which widely regulates neuronal excitability, although other subunits may contribute to M-like currents in some locations. M-channels are closed by receptors coupled to Gq such as M1 and M3 muscarinic receptors; this increases neuronal excitability and underlies some forms of cholinergic excitation. Muscarinic closure results from activation of phospholipase C and consequent hydrolysis and depletion of membrane phosphatidylinositol-4,5-bisphosphate, which is required for channel opening. Some effects of M-channel closure, determined from transmitter action, selective blocking drugs (linopirdine and XE991) and KCNQ2 gene disruption or manipulation, are as follows: (i) in sympathetic neurons: facilitation of repetitive discharges and conversion from phasic to tonic firing; (ii) in sensory nociceptive systems: facilitation of A-delta peripheral sensory fibre responses to noxious heat; and (iii) in hippocampal pyramidal neurons: facilitation of repetitive discharges, enhanced after-depolarization and burst-firing, and induction of spontaneous firing through a reduction of action potential threshold at the axon initial segment. Several drugs including flupirtine and retigabine enhance neural Kv7/M-channel activity, principally through a hyperpolarizing shift in their voltage gating. In consequence they reduce neural excitability and can inhibit nociceptive stimulation and transmission. Flupirtine is in use as a central analgesic; retigabine is under clinical trial as a broad-spectrum anticonvulsant and is an effective analgesic in animal models of chronic inflammatory and neuropathic pain.

Molecular domains of myelinated axons in the peripheral nervous system

Myelinated axons are organized into a series of specialized domains with distinct molecular compositions and functions. These domains, which include the node of Ranvier, the flanking paranodal junctions, the juxtaparanodes, and the internode, form as the result of interactions with myelinating Schwann cells. This domain organization is essential for action potential propagation by saltatory conduction and for the overall function and integrity of the axon.

Activation of pre-synaptic M-type K+ channels inhibits [3H]D-aspartate release by reducing Ca2+ entry through P/Q-type voltage-gated Ca2+ channels

In this study, the functional consequences of the pharmacological modulation of the M-current (I(KM)) on cytoplasmic Ca(2+) intracellular Ca(2+)concentration ([Ca(2+)](i)) changes and excitatory neurotransmitter release triggered by various stimuli from isolated rat cortical synaptosomes have been investigated. K(v)7.2 immunoreactivity was identified in pre-synaptic elements in cortical slices and isolated glutamatergic cortical synaptosomes. In cerebrocortical synaptosomes exposed to 20 mM [K(+)](e), the I(KM) activator retigabine (RT, 10 microM) inhibited [(3)H]D-aspartate ([(3)H]D-Asp) release and caused membrane hyperpolarization; both these effects were prevented by the I(KM) blocker XE-991 (20 microM). The I(KM) activators RT (0.1-30 microM), flupirtine (10 microM) and BMS-204352 (10 microM) inhibited 20 mM [K(+)](e)-induced synaptosomal [Ca(2+)](i) increases; XE-991 (20 microM) abolished RT-induced inhibition of depolarization-triggered [Ca(2+)](i) transients. The P/Q-type voltage-sensitive Ca(2+)channel (VSCC) blocker omega-agatoxin IVA prevented RT-induced inhibition of depolarization-induced [Ca(2+)](i) increase and [(3)H]D-Asp release, whereas the N-type blocker omega-conotoxin GVIA failed to do so. Finally, 10 microM RT did not modify the increase of [Ca(2+)](i) and the resulting enhancement of [(3)H]D-Asp release induced by [Ca(2+)](i) mobilization from intracellular stores, or by store-operated Ca(2+)channel activation. Collectively, the present data reveal that the pharmacological activation of I(KM) regulates depolarization-induced [(3)H]D-Asp release from cerebrocortical synaptosomes by selectively controlling the changes of [Ca(2+)](i) occurring through P/Q-type VSCCs.

Action potential initiation and propagation: upstream influences on neurotransmission

Axonal action potentials initiate the cycle of synaptic communication that is key to our understanding of nervous system functioning. The field has accumulated vast knowledge of the signature action potential waveform, firing patterns, and underlying channel properties of many cell types, but in most cases this information comes from somatic intracellular/whole-cell recordings, which necessarily measure a mixture of the currents compartmentalized in the soma, dendrites, and axon. Because the axon in many neuron types appears to be the site of lowest threshold for action potential initiation, the channel constellation in the axon is of particular interest. However, the axon is more experimentally inaccessible than the soma or dendrites. Recent studies have developed and applied single-fiber extracellular recording, direct intracellular recording, and optical recording techniques from axons toward understanding the behavior of the axonal action potential. We are starting to understand better how specific channels and other cellular properties shape action potential threshold, waveform, and timing: key elements contributing to downstream transmitter release. From this increased scrutiny emerges a theme of axons with more computational power than in traditional conceptualizations.

Modelled temperature-dependent excitability behaviour of a single ranvier node for a human peripheral sensory nerve fibre

The objective of this study was to determine whether the Hodgkin-Huxley model for unmyelinated nerve fibres could be modified to predict excitability behaviour at Ranvier nodes. Only the model parameters were modified to those of human, with the equations left unaltered. A model of a single Ranvier node has been developed as part of a larger model to describe excitation behaviour in a generalised human peripheral sensory nerve fibre. Parameter values describing the ionic and leakage conductances, corresponding equilibrium potentials, resting membrane potential and membrane capacitance of the original Hodgkin-Huxley model were modified to reflect the corresponding parameter values for human. Parameter temperature dependence was included. The fast activating potassium current kinetics were slowed down to represent those of a slow activating and deactivating potassium current, which do not inactivate. All calculations were performed in MATLAB. Action potential shape and amplitude were satisfactorily predicted at 20, 25 and 37 degrees C, and were not influenced by activation or deactivation of the slow potassium current. The calculated chronaxie time constant was 65.5 micros at 37 degrees C. However, chronaxie times were overestimated at temperatures lower than body temperature.

Protein kinase CK2 contributes to the organization of sodium channels in axonal membranes by regulating their interactions with ankyrin G

In neurons, generation and propagation of action potentials requires the precise accumulation of sodium channels at the axonal initial segment (AIS) and in the nodes of Ranvier through ankyrin G scaffolding. We found that the ankyrin-binding motif of Na(v)1.2 that determines channel concentration at the AIS depends on a glutamate residue (E1111), but also on several serine residues (S1112, S1124, and S1126). We showed that phosphorylation of these residues by protein kinase CK2 (CK2) regulates Na(v) channel interaction with ankyrins. Furthermore, we observed that CK2 is highly enriched at the AIS and the nodes of Ranvier in vivo. An ion channel chimera containing the Na(v)1.2 ankyrin-binding motif perturbed endogenous sodium channel accumulation at the AIS, whereas phosphorylation-deficient chimeras did not. Finally, inhibition of CK2 activity reduced sodium channel accumulation at the AIS of neurons. In conclusion, CK2 contributes to sodium channel organization by regulating their interaction with ankyrin G.

Ion channel clustering at the axon initial segment and node of Ranvier evolved sequentially in early chordates

In many mammalian neurons, dense clusters of ion channels at the axonal initial segment and nodes of Ranvier underlie action potential generation and rapid conduction. Axonal clustering of mammalian voltage-gated sodium and KCNQ (Kv7) potassium channels is based on linkage to the actin-spectrin cytoskeleton, which is mediated by the adaptor protein ankyrin-G. We identified key steps in the evolution of this axonal channel clustering. The anchor motif for sodium channel clustering evolved early in the chordate lineage before the divergence of the wormlike cephalochordate, amphioxus. Axons of the lamprey, a very primitive vertebrate, exhibited some invertebrate features (lack of myelin, use of giant diameter to hasten conduction), but possessed narrow initial segments bearing sodium channel clusters like in more recently evolved vertebrates. The KCNQ potassium channel anchor motif evolved after the divergence of lampreys from other vertebrates, in a common ancestor of shark and humans. Thus, clustering of voltage-gated sodium channels was a pivotal early innovation of the chordates. Sodium channel clusters at the axon initial segment serving the generation of action potentials evolved long before the node of Ranvier. KCNQ channels acquired anchors allowing their integration into pre-existing sodium channel complexes at about the same time that ancient vertebrates acquired myelin, saltatory conduction, and hinged jaws. The early chordate refinements in action potential mechanisms we have elucidated appear essential to the complex neural signaling, active behavior, and evolutionary success of vertebrates.

[Axon and Schwann cells... so far away, so close]

Myelination was a major step in the evolution of the nervous system. Appearing first in jaw fish, myelination allows the fast and secure propagation of action potentials at a low energetic cost, and without exaggerated increase in axonal diameter. In the peripheral nervous system of mammals, myelination results from the tight interactions between Schwann cells and axons, leading to the formation of highly differentiated domains along the axon. The molecular determinants of these interactions are starting to be well identified. Their understanding provides a precise framework to interpret the defects, which occur in pathological circumstances. This review summarizes the present state of knowledge concerning axoglial interactions in peripheral nerves.