Polarized distribution of ion channels within microdomains of the axon initial segment

Voltage-gated Na(+) channels in chemoreceptor afferent neurons--potential roles and changes with development

Carotid body chemoreceptors increase their action potential (AP) activity in response to a decrease in arterial oxygen tension and this response increases in the post-natal period. The initial transduction site is likely the glomus cell which responds to hypoxia with an increase in intracellular calcium and secretion of multiple neurotransmitters. Translation of this secretion to AP spiking levels is determined by the excitability of the afferent nerve terminals that is largely determined by the voltage-dependence of activation of Na(+) channels. In this review, we examine the biophysical characteristics of Na(+) channels present at the soma of chemoreceptor afferent neurons with the assumption that similar channels are present at nerve terminals. The voltage dependence of this current is consistent with a single Na(+) channel isoform with activation around the resting potential and with about 60-70% of channels in the inactive state around the resting potential. Channel openings, due to transitions from inactive/open or closed/open states, may serve to amplify external depolarizing events or generate, by themselves, APs. Over the first two post-natal weeks, the Na(+) channel activation voltage shifts to more negative potentials, thus enhancing the amplifying action of Na(+) channels on depolarization events and increasing membrane noise generated by channel transitions. This may be a significant contributor to maturation of chemoreceptor activity in the post-natal period.

Undersized dendritic arborizations in retinal ganglion cells of the rd1 mutant mouse: a paradigm of early onset photoreceptor degeneration

Retinitis pigmentosa (RP) is a family of inherited diseases causing progressive photoreceptor death. Retinal ganglion cells (RGCs) form the biological substrate for various therapeutic approaches designed to restore vision in RP individuals. Assessment of survival and preservation of RGCs in animal paradigms mimicking the human disease is of key importance for appropriate implementation of vision repair strategies. Here we studied the survival of RGCs in the rd1 mutant mouse, a known model of early onset, autosomic recessive RP, at various stages of photoreceptor degeneration. Furthermore, we analyzed the morphology of various types of RGCs using the newly generated transgenic mouse rd1/Thy1-GFP, in which the rd1 mutation is associated with green fluorescent protein (GFP) expression in a small population of different RGCs. We found excellent survival of cells at up to 1 year of age, a time at which the inner retina is known to have severely reorganized and partially degenerated. However, 50% of the cells analyzed within all RGC types exhibit an undersized dendritic tree, spanning about half of the normal area. Undersized cells are found both in adult and in very young (1-month-old) mice. This suggests that their aberrant phenotype is due to incomplete dendritic development, possibly as a consequence of altered visual input at the time of dendritic arbor refinement. These data show the importance of the timing of photoreceptor death in RGC dendritic development.

Kv1.1 and Kv1.2: similar channels, different seizure models

Voltage-gated K(+) channels (Kv) represent the largest family of genes in the K(+) channel family. The Kv1 subfamily plays an essential role in the initiation and shaping of action potentials, influencing action potential firing patterns and controlling neuronal excitability. Overlapping patterns with differential expression and precise localization of Kv1.1 and Kv1.2 channels targeted to specialized subcellular compartments contribute to distinctive patterns of neuronal excitability. Dynamic regulation of the components in these subcellular domains help to finely tune the cellular and regional networks. Disruption of the expression, distribution, and density of these channels through deletion or mutation of the genes encoding these channels, Kcna1 and Kcna2, is associated with neurologic pathologies including epilepsy and ataxia in humans and in rodent models. Kv1.1 and Kv1.2 knockout mice both have seizures beginning early in development; however, each express a different seizure type (pathway), although the channels are from the same subfamily and are abundantly coexpressed. Voltage-gated ion channels clustered in specific locations may present a novel therapeutic target for influencing excitability in neurologic disorders associated with some channelopathies.

Synthesis and biological evaluation of a fluorescent analog of phenytoin as a potential inhibitor of neuropathic pain and imaging agent

Here we report on a novel fluorescent analog of phenytoin as a potential inhibitor of neuropathic pain with potential use as an imaging agent. Compound 2 incorporated a heptyl side chain and dansyl moiety onto the parent compound phenytoin and produced greater displacement of BTX from sodium channels and greater functional blockade with greatly reduced toxicity. Compound 2 reduced mechano-allodynia in a rat model of neuropathic pain and was visualized ex vivo in sensory neuron axons with two-photon microscopy. These results suggest a promising strategy for developing novel sodium channel inhibitors with imaging capabilities.

Increased Kv1 channel expression may contribute to decreased sIPSC frequency following chronic inhibition of NR2B-containing NMDAR

Numerous studies have documented the effects of chronic N-methyl-D-aspartate receptor (NMDAR) blockade on excitatory circuits, but the effects on inhibitory circuitry are not well studied. NR2A- and NR2B-containing NMDARs play differential roles in physiological processes, but the consequences of chronic NR2A- or NR2B-containing NMDAR inhibition on glutamatergic and GABAergic neurotransmission are unknown. We investigated altered GABAergic neurotransmission in dentate granule cells and interneurons following chronic treatment with the NR2B-selective antagonist, Ro25,6981, the NR2A-prefering antagonist, NVP-AAM077, or the non-subunit-selective NMDAR antagonist, D-APV, in organotypic hippocampal slice cultures. Electrophysiological recordings revealed large reductions in spontaneous inhibitory postsynaptic current (sIPSC) frequency in both granule cells and interneurons following chronic Ro25,6981 treatment, which was associated with minimally altered sIPSC amplitude, miniature inhibitory postsynaptic current (mIPSC) frequency, and mIPSC amplitude, suggesting diminished action potential-dependent GABA release. Chronic NVP-AAM077 or D-APV treatment had little effect on these measures. Reduced sIPSC frequency did not arise from downregulated GABA(A)R, altered excitatory or inhibitory drive to interneurons, altered interneuron membrane properties, increased failure rate, decreased action potential-dependent release probability, or mGluR/GABA(B) receptor modulation of GABA release. However, chronic Ro25,6981-mediated reductions in sIPSC frequency were occluded by the K+ channel blockers, dendrotoxin, margatoxin, and agitoxin, but not dendrotoxin-K or XE991. Immunohistochemistry also showed increased Kv1.2, Kv1.3, and Kv1.6 in the dentate molecular layer following chronic Ro25,6981 treatment. Our findings suggest that increased Kv1 channel expression/function contributed to diminished action potential-dependent GABA release following chronic NR2B-containing NMDAR inhibition and that these Kv1 channels may be heteromeric complexes containing Kv1.2, Kv1.3, and Kv1.6.

Characterization of axo-axonic synapses in the piriform cortex of Mus musculus

Previous anatomical and physiological studies have established major glutamatergic and GABAergic neuronal subtypes within the piriform cortical circuits. However, quantitative information regarding axo-axonic inhibitory synapses mediated by chandelier cells across major cortical subdivisions of piriform cortex is lacking. Therefore, we examined the properties of these synapses across the entire piriform cortex. Our results show the following. 1) γ-Aminobutyric acid membrane transporter 1-positive varicosities, whose appearance resembles chandelier cartridges, are found around the initial segments of axons of glutamatergic cells across layers II and III. 2) Both the density of axo-axonic cartridges and the degree of γ-aminobutyric acid membrane transporter 1 innervation in each axo-axonic synapse are significantly higher in the piriform cortex than in the neocortex. 3) Glutamate decarboxylase 67, vesicular GABA transporter, and parvalbumin, but not calbindin, are colocalized with the presynaptic varicosities, whereas gephyrin, Na-K-2Cl cotransporter 1, and GABA(A) receptor α1 subunit, but not K-Cl cotransporter 2, are colocalized at the presumed postsynaptic sites. 4) The axo-axonic cartridges innervate the majority of excitatory neurons and are distributed more frequently in putative centrifugal cells and posterior piriform cortex. We further describe the morphology of chandelier cells by using parvalbumin-immunoreactivity and single-cell labeling. In summary, our results demonstrate that a small population of chandelier cells mediates abundant axo-axonic synapses across the entire piriform cortex. Because of the critical location of these inhibitory synapses in relation to action potential regulation, our results highlight a critical role of axo-axonic synapses in regulating information flow and olfactory-related oscillations within the piriform cortex in vivo.

Voltage-gated sodium channel-associated proteins and alternative mechanisms of inactivation and block

Voltage-gated sodium channels mediate inward current of action potentials upon membrane depolarization of excitable cells. The initial transient sodium current is restricted to milliseconds through three distinct channel-inactivating and blocking mechanisms. All pore-forming alpha subunits of sodium channels possess structural elements mediating fast inactivation upon depolarization and recovery within milliseconds upon membrane repolarization. Accessory subunits modulate fast inactivation dynamics, but these proteins can also limit current by contributing distinct inactivation and blocking particles. A-type isoforms of fibroblast growth factor homologous factors (FHFs) bear a particle that induces long-term channel inactivation, while sodium channel subunit Navβ4 employs a blocking particle that rapidly dissociates upon membrane repolarization to generate resurgent current. Despite their different physiological functions, the FHF and Navβ4 particles have similarity in amino acid composition and mechanisms for docking within sodium channels. The three competing channel-inactivating and blocking processes functionally interact to regulate a neuron's intrinsic excitability.

Modulation of neuronal sodium channels by the sea anemone peptide BDS-I

Blood-depressing substance I (BDS-I), a 43 amino-acid peptide from sea anemone venom, is used as a specific inhibitor of Kv3-family potassium channels. We found that BDS-I acts with even higher potency to modulate specific types of voltage-dependent sodium channels. In rat dorsal root ganglion (DRG) neurons, 3 μM BDS-I strongly enhanced tetrodotoxin (TTX)-sensitive sodium current but weakly inhibited TTX-resistant sodium current. In rat superior cervical ganglion (SCG) neurons, which express only TTX-sensitive sodium current, BDS-I enhanced current elicited by small depolarizations and slowed decay of currents at all voltages (EC(50) ∼ 300 nM). BDS-I acted with exceptionally high potency and efficacy on cloned human Nav1.7 channels, slowing inactivation by 6-fold, with an EC(50) of approximately 3 nM. BDS-I also slowed inactivation of sodium currents in N1E-115 neuroblastoma cells (mainly from Nav1.3 channels), with an EC(50) ∼ 600 nM. In hippocampal CA3 pyramidal neurons (mouse) and cerebellar Purkinje neurons (mouse and rat), BDS-I had only small effects on current decay (slowing inactivation by 20-50%), suggesting relatively weak sensitivity of Nav1.1 and Nav1.6 channels. The biggest effect of BDS-I in central neurons was to enhance resurgent current in Purkinje neurons, an effect reflected in enhancement of sodium current during the repolarization phase of Purkinje neuron action potentials. Overall, these results show that BDS-I acts to modulate sodium channel gating in a manner similar to previously known neurotoxin receptor site 3 anemone toxins but with different isoform sensitivity. Most notably, BDS-I acts with very high potency on human Nav1.7 channels.

The physiology of the axon initial segment

The action potential generally begins in the axon initial segment (AIS), a principle confirmed by 60 years of research; however, the most recent advances have shown that a very rich biology underlies this simple observation. The AIS has a remarkably complex molecular composition, with a wide variety of ion channels and attendant mechanisms for channel localization, and may feature membrane domains each with distinct roles in excitation. Its function may be regulated in the short term through the action of neurotransmitters, in the long term through activity- and Ca(2+)-dependent processes. Thus, the AIS is not merely the beginning of the axon, but rather a key site in the control of neuronal excitability.

A-kinase anchoring protein 150 expression in a specific subset of TRPV1- and CaV 1.2-positive nociceptive rat dorsal root ganglion neurons

Modulation of phosphorylation states of ion channels is a critical step in the development of hyperalgesia during inflammation. Modulatory enhancement of channel activity may increase neuronal excitability and affect downstream targets such as gene transcription. The specificity required for such regulation of ion channels quickly occurs via targeting of protein kinases and phosphatases by the scaffolding A-kinase anchoring protein 79/150 (AKAP79/150). AKAP79/150 has been implicated in inflammatory pain by targeting protein kinase A (PKA) and protein kinase C (PKC) to the transient receptor potential vanilloid 1 (TRPV1) channel in peripheral sensory neurons, thus lowering threshold for activation of the channel by multiple inflammatory reagents. However, the expression pattern of AKAP150 in peripheral sensory neurons is unknown. Here we identify the peripheral neuron subtypes that express AKAP150, the subcellular distribution of AKAP150, and the potential target ion channels in rat dorsal root ganglion (DRG) slices. We found that AKAP150 is expressed predominantly in a subset of small DRG sensory neurons, where it is localized at the plasma membrane of the soma, axon initial segment, and small fibers. Most of these neurons are peripherin positive and produce C fibers, although a small portion produce Aδ fibers. Furthermore, we demonstrate that AKAP79/150 colocalizes with TRPV1 and Ca(V) 1.2 in the soma and axon initial segment. Thus AKAP150 is expressed in small, nociceptive DRG neurons, where it is targeted to membrane regions and where it may play a role in the modulation of ion channel phosphorylation states required for hyperalgesia.

Differential subcellular distribution of ion channels and the diversity of neuronal function

Following the astonishing molecular diversity of voltage-gated ion channels that was revealed in the past few decades, the ion channel repertoire expressed by neurons has been implicated as the major factor governing their functional heterogeneity. Although the molecular structure of ion channels is a key determinant of their biophysical properties, their subcellular distribution and densities on the surface of nerve cells are just as important for fulfilling functional requirements. Recent results obtained with high resolution quantitative localization techniques revealed complex, subcellular compartment-specific distribution patterns of distinct ion channels. Here I suggest that within a given neuron type every ion channel has a unique cell surface distribution pattern, with the functional consequence that this dramatically increases the computational power of nerve cells.

Characterization of the axon initial segment (AIS) of motor neurons and identification of a para-AIS and a juxtapara-AIS, organized by protein 4.1B

Background

The axon initial segment (AIS) plays a crucial role: it is the site where neurons initiate their electrical outputs. Its composition in terms of voltage-gated sodium (Nav) and voltage-gated potassium (Kv) channels, as well as its length and localization determine the neuron's spiking properties. Some neurons are able to modulate their AIS length or distance from the soma in order to adapt their excitability properties to their activity level. It is therefore crucial to characterize all these parameters and determine where the myelin sheath begins in order to assess a neuron's excitability properties and ability to display such plasticity mechanisms. If the myelin sheath starts immediately after the AIS, another question then arises as to how would the axon be organized at its first myelin attachment site; since AISs are different from nodes of Ranvier, would this particular axonal region resemble a hemi-node of Ranvier?

Results

We have characterized the AIS of mouse somatic motor neurons. In addition to constant determinants of excitability properties, we found heterogeneities, in terms of AIS localization and Nav composition. We also identified in all α motor neurons a hemi-node-type organization, with a contactin-associated protein (Caspr)⁺ paranode-type, as well as a Caspr2⁺ and Kv1⁺ juxtaparanode-type compartment, referred to as a para-AIS and a juxtapara (JXP)-AIS, adjacent to the AIS, where the myelin sheath begins. We found that Kv1 channels appear in the AIS, para-AIS and JXP-AIS concomitantly with myelination and are progressively excluded from the para-AIS. Their expression in the AIS and JXP-AIS is independent from transient axonal glycoprotein-1 (TAG-1)/Caspr2, in contrast to juxtaparanodes, and independent from PSD-93. Data from mice lacking the cytoskeletal linker protein 4.1B show that this protein is necessary to form the Caspr⁺ para-AIS barrier, ensuring the compartmentalization of Kv1 channels and the segregation of the AIS, para-AIS and JXP-AIS.

Conclusions

α Motor neurons have heterogeneous AISs, which underlie different spiking properties. However, they all have a para-AIS and a JXP-AIS contiguous to their AIS, where the myelin sheath begins, which might limit some AIS plasticity. Protein 4.1B plays a key role in ensuring the proper molecular compartmentalization of this hemi-node-type region.

Cdk5 regulates the size of an axon initial segment-like compartment in mushroom body neurons of the Drosophila central brain

The axon initial segment (AIS) is the specialized compartment of vertebrate axons where action potentials are initiated. Despite longtime attention to the unique functions of this compartment, the mechanisms that regulate AIS formation and maintenance are not known. Here, we identify a novel compartment in Drosophila mushroom body neurons that mirrors the molecular hallmarks of the vertebrate AIS as judged by accumulation of the anchoring protein Ankyrin1, presence of a specialized actin cytoskeleton, exclusion of both axon-specific and somatodendritic-specific cell surface proteins, and accumulation of a unique combination of voltage-gated ion channels. Using pharmacological treatments, we show that, similar to the vertebrate AIS, the integrity of this region of γ-neurons and its ability to tether membrane proteins depends on an intact actin cytoskeleton. We further show that Cdk5/p35 kinase regulates the formation and maintenance of the putative AIS by controlling the position of its distal boundary. Thus, boosting Cdk5 activity in γ-neurons extends the AIS by as much as 100%, while eliminating Cdk5 activity causes the domain to shrink proximally or disappear altogether. These data demonstrate that Cdk5/p35 kinase is a key regulator of the development and maintenance of the AIS in Drosophila.

Maintenance of neuronal polarity

Neurons are highly polarized cells with distinct domains responsible for receiving, transmitting, and propagating electrical signals. Central to these functions is the axon initial segment (AIS), a short region of the axon adjacent to the cell body that is enriched in voltage-gated ion channels, cell adhesion molecules, and cytoskeletal scaffolding proteins. Traditionally, the function of the AIS has been limited to its role in action potential initiation and modulation. However, recent experiments indicate that it also plays essential roles in neuronal polarity. Here, we review how initial segments are assembled, and discuss proposed mechanisms for how the AIS contributes to maintenance of neuronal polarity.

Dendrite-derived supernumerary axons on adult axotomized motor neurons possess proteins that are essential for the initiation and propagation of action potentials and synaptic vesicle release

Axotomy can trigger profound alterations in the neuronal polarity of adult neurons in vivo. This can manifest itself in the development of new axon-like processes emanating from the tips of distal dendrites. Previously, these processes have been defined as axonal based on their axonal morphology. This study extends this definition to determine whether, more importantly, these processes possess the prerequisite molecular machinery to function as axons. Using a combination of intracellular labeling and immunohistochemistry, we demonstrate that the distribution of voltage-gated sodium channels on these processes matches the arrangement of these channels that is necessary for the initiation and conduction of action potentials. At terminal bouton-like structures they possess key proteins necessary for the release of synaptic vesicles (SV2 and synaptophysin). Thus, axon-like processes emanating from the tips of distal dendrites represent a rearrangement of neuronal polarity whereby axotomized neurons can develop additional functional axons in vivo.

Response variability to high rates of electric stimulation in retinal ganglion cells

To improve the quality of prosthetic vision, it is important to understand how retinal neurons respond to electric stimulation. Previous studies present conflicting reports as to the maximum rate at which retinal ganglion cells can "follow" pulse trains, i.e., generate one spike for each pulse of the train. In the present study, we measured the response of 5 different types of rabbit retinal ganglion cells to pulse trains of 100-700 Hz. Surprisingly, we found significant heterogeneity in the ability of different types to follow pulse trains. For example, brisk transient (BT) ganglion cells could reliably follow pulse rates up to 600 pulses per second (PPS). In contrast, other types could not even follow rates of 200 PPS. There was additional heterogeneity in the response patterns across those types that could not follow high-rate trains. For example, some types generated action potentials in response to approximately every other pulse, whereas other types generated one spike per pulse for a few consecutive pulses and then did not generate any spikes in response to the next few pulses. Interestingly, in the types that could not follow high-rate trains, we found a second type of response: many pulses of the train elicited a biphasic waveform with an amplitude much smaller than that of standard action potentials. This small waveform was often observed following every pulse for which a standard spike was not elicited. A possible origin of the small waveform and its implication for effective retinal stimulation are discussed.

The spatio-temporal characteristics of action potential initiation in layer 5 pyramidal neurons: a voltage imaging study

The spatial pattern of Na(+) channel clustering in the axon initial segment (AIS) plays a critical role in tuning neuronal computations, and changes in Na(+) channel distribution have been shown to mediate novel forms of neuronal plasticity in the axon. However, immunocytochemical data on channel distribution may not directly predict spatio-temporal characteristics of action potential initiation, and prior electrophysiological measures are either indirect (extracellular) or lack sufficient spatial resolution (intracellular) to directly characterize the spike trigger zone (TZ). We took advantage of a critical methodological improvement in the high sensitivity membrane potential imaging (V(m) imaging) technique to directly determine the location and length of the spike TZ as defined in functional terms. The results show that in mature axons of mouse cortical layer 5 pyramidal cells, action potentials initiate in a region ∼20 μm in length centred between 20 and 40 μm from the soma. From this region, the AP depolarizing wave invades initial nodes of Ranvier within a fraction of a millisecond and propagates in a saltatory fashion into axonal collaterals without failure at all physiologically relevant frequencies. We further demonstrate that, in contrast to the saltatory conduction in mature axons, AP propagation is non-saltatory (monotonic) in immature axons prior to myelination.

The sodium channel band shapes the response to electric stimulation in retinal ganglion cells

To improve the quality of prosthetic vision, it is desirable to understand how targeted retinal neurons respond to stimulation. Unfortunately, the factors that shape the response of a single neuron to stimulation are not well understood. A dense band of voltage-gated sodium channels within the proximal axon of retinal ganglion cells is the site most sensitive to electric stimulation, suggesting that band properties are likely to influence the response to stimulation. Here, we examined how three band properties influence sensitivity using a morphologically realistic ganglion cell model in NEURON. Longer bands were more sensitive to short-duration pulses than shorter bands and increasing the distance between band and soma also increased sensitivity. Simulations using the known limits of band length and location resulted in a sensitivity difference of approximately 2. Additional simulations tested how changes to sodium channel conductance within the band influenced threshold and found that the sensitivity difference increased to a factor of nearly 3. This is close to the factor of 5 difference measured in physiological studies suggesting that band properties contribute significantly to the sensitivity differences found between different types of retinal neurons.

Neuronal trafficking of voltage-gated potassium channels

The computational ability of CNS neurons depends critically on the specific localization of ion channels in the somatodendritic and axonal membranes. Neuronal dendrites receive synaptic inputs at numerous spines and integrate them in time and space. The integration of synaptic potentials is regulated by voltage-gated potassium (Kv) channels, such as Kv4.2, which are specifically localized in the dendritic membrane. The synaptic potentials eventually depolarize the membrane of the axon initial segment, thereby activating voltage-gated sodium channels to generate action potentials. Specific Kv channels localized in the axon initial segment, such as Kv1 and Kv7 channels, determine the shape and the rate of action potentials. Kv1 and Kv7 channels present at or near nodes of Ranvier and in presynaptic terminals also influence the propagation of action potentials and neurotransmitter release. The physiological significance of proper Kv channel localization is emphasized by the fact that defects in the trafficking of Kv channels are observed in several neurological disorders including epilepsy. In this review, we will summarize the current understanding of the mechanisms of Kv channel trafficking and discuss how they contribute to the establishment and maintenance of the specific localization of Kv channels in neurons.

A critical role for Neurofascin in regulating action potential initiation through maintenance of the axon initial segment

The axon initial segment (AIS) is critical for the initiation and propagation of action potentials. Assembly of the AIS requires interactions between scaffolding molecules and voltage-gated sodium channels, but the molecular mechanisms that stabilize the AIS are poorly understood. The neuronal isoform of Neurofascin, Nfasc186, clusters voltage-gated sodium channels at nodes of Ranvier in myelinated nerves: here, we investigate its role in AIS assembly and stabilization. Inactivation of the Nfasc gene in cerebellar Purkinje cells of adult mice causes rapid loss of Nfasc186 from the AIS but not from nodes of Ranvier. This causes AIS disintegration, impairment of motor learning and the abolition of the spontaneous tonic discharge typical of Purkinje cells. Nevertheless, action potentials with a modified waveform can still be evoked and basic motor abilities remain intact. We propose that Nfasc186 optimizes communication between mature neurons by anchoring the key elements of the adult AIS complex.

Physiological synaptic signals initiate sequential spikes at soma of cortical pyramidal neurons

The neurons in the brain produce sequential spikes as the digital codes whose various patterns manage well-organized cognitions and behaviors. A source for the physiologically integrated synaptic signals to initiate digital spikes remains unknown, which we studied at pyramidal neurons of cortical slices. In dual recordings from the soma vs. axon, the signals recorded in vivo induce somatic spikes with higher capacity, which is associated with lower somatic thresholds and shorter refractory periods mediated by voltage-gated sodium channels. The introduction of these parameters from the soma and axon into NEURON model simulates sequential spikes being somatic in origin. Physiological signals integrated from synaptic inputs primarily trigger the soma to encode neuronal digital spikes.

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.

The developmental changes of Na(v)1.1 and Na(v)1.2 expression in the human hippocampus and temporal lobe

Alterations of the genes encoding α1 and α2 subunits of voltage-gated sodium channels (SCN1A, SCN2A) have been reported as causes of various types of epilepsy, most of which occur during the first year of life; as yet, however, the detailed mechanisms are unclear. We suppose that developmental changes of SCN1A and SCN2A in the human brain, which are unknown yet, may play an important role. So here, we studied the developmental changes of their corresponding proteins (Na(v)1.1 and Na(v)1.2) in the human hippocampus and temporal lobe in 28 autopsy cases, which age from 13weeks of gestation (GW) to 63years of age (Y). Using comparative microscopic immunohistochemical (IHC) analysis, we found that Na(v)1.1 and Na(v)1.2 immunoreactivity first appeared at 19GW, simultaneously in the hippocampus and the white matter of temporal lobe. In nearly all age groups, Na(v)1.1 immunoreactivity was weak and relatively homogeneous. In general, Na(v)1.1 immunoreactive (IR) neurons and neurites increased during the late fetal and postnatal periods, reached their peaks 7-9months after birth (M), then decreased and remained stable at a relatively low level during childhood and adulthood. On the other hand, Na(v)1.2 immunoreactivity was strong and heterogeneous. In the hippocampus, Na(v)1.2 IR neurons increased gradually during the late fetal period, reached their peaks at 7-9M, sustained this high level during childhood, and then decreased slightly at adulthood. In the temporal lobe, Na(v)1.2 IR neurons reached a high level during the late fetal period, and maintained that level during subsequent developmental stages; Na(v)1.2 IR neurites also increased to a relatively high level during the late fetal period and continued to increase up to and during adulthood. Using double-staining IHC, we found that Na(v)1.1 and Na(v)1.2 had a relatively high colocalization rate with parvalbumin and showed distinct developmental changes. These findings extend our previous understanding of sodium channels and may help us discover the pathomechanisms of sodium channel-related age-dependent epilepsy.

Cdk-mediated phosphorylation of the Kvβ2 auxiliary subunit regulates Kv1 channel axonal targeting

Phosphorylation of Kvβ2 releases Kv1 channels from microtubules to control their specific distribution at the axonal membrane.

Kv1 channels are concentrated at specific sites in the axonal membrane, where they regulate neuronal excitability. Establishing these distributions requires regulated dissociation of Kv1 channels from the neuronal trafficking machinery and their subsequent insertion into the axonal membrane. We find that the auxiliary Kvβ2 subunit of Kv1 channels purified from brain is phosphorylated on serine residues 9 and 31, and that cyclin-dependent kinase (Cdk)–mediated phosphorylation at these sites negatively regulates the interaction of Kvβ2 with the microtubule plus end–tracking protein EB1. Endogenous Cdks, EB1, and Kvβ2 phosphorylated at serine 31 are colocalized in the axons of cultured hippocampal neurons, with enrichment at the axon initial segment (AIS). Acute inhibition of Cdk activity leads to intracellular accumulation of EB1, Kvβ2, and Kv1 channel subunits within the AIS. These studies reveal a new regulatory mechanism for the targeting of Kv1 complexes to the axonal membrane through the reversible Cdk phosphorylation-dependent binding of Kvβ2 to EB1.

Expansion of voltage-dependent Na+ channel gene family in early tetrapods coincided with the emergence of terrestriality and increased brain complexity

Mammals have ten voltage-dependent sodium (Nav) channel genes. Nav channels are expressed in different cell types with different subcellular distributions and are critical for many aspects of neuronal processing. The last common ancestor of teleosts and tetrapods had four Nav channel genes, presumably on four different chromosomes. In the lineage leading to mammals, a series of tandem duplications on two of these chromosomes more than doubled the number of Nav channel genes. It is unknown when these duplications occurred and whether they occurred against a backdrop of duplication of flanking genes on their chromosomes or as an expansion of ion channel genes in general. We estimated key dates of the Nav channel gene family expansion by phylogenetic analysis using teleost, elasmobranch, lungfish, amphibian, avian, lizard, and mammalian Nav channel sequences, as well as chromosomal synteny for tetrapod genes. We tested, and exclude, the null hypothesis that Nav channel genes reside in regions of chromosomes prone to duplication by demonstrating the lack of duplication or duplicate retention of surrounding genes. We also find no comparable expansion in other voltage-dependent ion channel gene families of tetrapods following the teleost-tetrapod divergence. We posit a specific expansion of the Nav channel gene family in the Devonian and Carboniferous periods when tetrapods evolved, diversified, and invaded the terrestrial habitat. During this time, the amniote forebrain evolved greater anatomical complexity and novel tactile sensory receptors appeared. The duplication of Nav channel genes allowed for greater regional specialization in Nav channel expression, variation in subcellular localization, and enhanced processing of somatosensory input.

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.

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.

Dendritic spikes amplify the synaptic signal to enhance detection of motion in a simulation of the direction-selective ganglion cell

The On-Off direction-selective ganglion cell (DSGC) in mammalian retinas responds most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree, which are thought to propagate to the soma with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas somatic PSPs (postsynaptic potentials) are only weakly directional, indicating that spike generation includes marked enhancement of the directional signal. We used a realistic computational model based on anatomical and physiological measurements to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate electrotonic regions, each summing its local excitatory and inhibitory synaptic inputs to initiate spikes. Within each local region the local spike threshold nonlinearly amplifies the preferred response over the null response on the basis of PSP amplitude. Using inhibitory conductances previously measured in DSGCs, the simulation results showed that inhibition is only sufficient to prevent spike initiation and cannot affect spike propagation. Therefore, inhibition will only act locally within the dendritic arbor. We identified the role of three mechanisms that generate directional selectivity (DS) in the local dendritic regions. First, a mechanism for DS intrinsic to the dendritic structure of the DSGC enhances DS on the null side of the cell's dendritic tree and weakens it on the preferred side. Second, spatially offset postsynaptic inhibition generates robust DS in the isolated dendritic tips but weak DS near the soma. Third, presynaptic DS is apparently necessary because it is more robust across the dendritic tree. The pre- and postsynaptic mechanisms together can overcome the local intrinsic DS. These local dendritic mechanisms can perform independent nonlinear computations to make a decision, and there could be analogous mechanisms within cortical circuitry.

The On-Off direction-selective ganglion cell (DSGC) found in mammalian retinas generates a directional signal, responding most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree which are thought to propagate to the soma and brain with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas postsynaptic potentials (PSPs) recorded in the soma are only weakly directional, indicating that postsynaptic spike generation markedly enhances the directional signal. We constructed a realistic computational model to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate computational regions. Within each region, the local spike threshold produces nonlinear amplification of the preferred response over the null response on the basis of PSP amplitude. The simulation results showed that inhibition acts locally within the dendritic arbor and will not stop dendritic spikes from propagating. We identified the role of three mechanisms that generate direction selectivity in the local dendritic regions, which suggests the origin of the previously described “non-direction-selective region,” and also suggests that the known DS in the synaptic inputs is apparently necessary for robust DS across the dendritic tree.

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.

Building and maintaining the axon initial segment

The axon initial segment is a unique neuronal subregion involved in the initiation of action potentials and in the control of axonal identity. Recent work has helped our understanding of how this specialised structure develops, not least in identifying possible mechanisms leading to the localisation of the AIS's master organiser protein, ankyrin-G. The most exciting current work, however, focuses on later aspects of AIS function and plasticity. Recent studies have shown that the AIS is subdivided into distinct structural and functional domains, have demonstrated how the AIS acts as a cytoplasmic barrier for axonal transport, and have discovered that the AIS can be surprisingly plastic in its responses to alterations in neuronal activity.

The axon initial segment and the maintenance of neuronal polarity

Ion channel clustering at the axon initial segment (AIS) and nodes of Ranvier has been suggested to be a key evolutionary innovation that enabled the development of the complex vertebrate nervous system. This innovation epitomizes a signature feature of neurons, namely polarity. The mechanisms that establish neuronal polarity, channel clustering and axon-dendrite identity during development are becoming clearer. However, much less is known about how polarity is maintained throughout life. Here, I review the role of the AIS in the development and maintenance of neuronal polarity and discuss how disrupted polarity may be a common component of many diseases and injuries that affect the nervous system.

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.

Multisite phosphorylation of voltage-gated sodium channel alpha subunits from rat brain

Reversible phosphorylation of ion channels underlies cellular plasticity in mammalian neurons. Voltage-gated sodium or Nav channels underlie action potential initiation and propagation, dendritic excitability, and many other aspects of neuronal excitability. Various protein kinases have been suggested to phosphorylate the primary or alpha subunit of Nav channels, affecting diverse aspects of channel function. Previous studies of Nav alpha subunit phosphorylation have led to the identification of a small set of phosphorylation sites important in mediating diverse aspects of Nav channel function. Here we use nanoflow liquid chromatography tandem mass spectrometry (nano-LC MS/MS) on Nav alpha subunits affinity-purified from rat brain with two distinct monoclonal antibodies to identify 15 phosphorylation sites on Nav1.2, 12 of which have not been previously reported. We also found 3 novel phosphorylation sites on Nav1.1. In general, commonly used phosphorylation site prediction algorithms did not accurately predict these novel in vivo phosphorylation sites. Our results demonstrate that specific Nav alpha subunits isolated from rat brain are highly phosphorylated, and suggest extensive modulation of Nav channel activity in mammalian brain. Identification of phosphorylation sites using monoclonal antibody-based immunopurification and mass spectrometry is an effective approach to define the phosphorylation status of Nav channels and other important membrane proteins in mammalian brain.

To depolarize or hyperpolarize? at the axon initial segment, EGABA sets the stage

GABAergic Depolarization of the Axon Initial Segment in Cortical Principal Neurons Is Caused by the Na-K-2Cl Cotransporter NKCC1. Khirug S, Yamada J, Afzalov R, Voipio J, Khiroug L, Kaila K. J Neurosci. 2008;28(18):4635–4639. GABAergic terminals of axo-axonic cells (AACs) are exclusively located on the axon initial segment (AIS) of cortical principal neurons, and they are generally thought to exert a powerful inhibitory action. However, recent work (Szabadics et al., 2006) indicates that this input from AACs can be depolarizing and even excitatory. Here, we used local photolysis of caged GABA to measure reversal potentials ( EGABA) of GABAA receptor-mediated currents and to estimate the local chloride concentration in the AIS compared with other cellular compartments in dentate granule cells and neocortical pyramidal neurons. We found a robust axo-somato-dendritic –gradient in which the EGABA values from the AIS to the soma and dendrites become progressively more negative. Data from NKCC1-/- and bumetanide-exposed neurons indicated that the depolarizing EGABA at the AIS is set by chloride uptake mediated by the Na–K–2Cl cotransporter NKCC1. Our findings demonstrate that spatially distinct interneuronal inputs can induce postsynaptic voltage responses with different amplitudes and polarities as governed by the subcellular distributions of plasmalemmal chloride transporters.

Axonal sodium channel distribution shapes the depolarized action potential threshold of dentate granule neurons

Intrinsic excitability is a key feature dictating neuronal response to synaptic input. Here we investigate the recent observation that dentate granule neurons exhibit a more depolarized voltage threshold for action potential initiation than CA3 pyramidal neurons. We find no evidence that tonic GABA currents, leak or voltage-gated potassium conductances, or the expression of sodium channel isoform differences can explain this depolarized threshold. Axonal initial segment voltage-gated sodium channels, which are dominated by the Na(V)1.6 isoform in both cell types, distribute more proximally and exhibit lower overall density in granule neurons than in CA3 neurons. To test possible contributions of sodium channel distributions to voltage threshold and to test whether morphological differences participate, we performed simulations of dentate granule neurons and of CA3 pyramidal neurons. These simulations revealed that cell morphology and sodium channel distribution combine to yield the characteristic granule neuron action potential upswing and voltage threshold. Proximal axon sodium channel distribution strongly contributes to the higher voltage threshold of dentate granule neurons for two reasons. First, action potential initiation closer to the somatodendritic current sink causes the threshold of the initiating axon compartment to rise. Second, the proximity of the action potential initiation site to the recording site causes somatic recordings to more faithfully reflect the depolarized threshold of the axon than in cells like CA3 neurons, with distally initiating action potentials. Our results suggest that the proximal location of axon sodium channels in dentate granule neurons contributes to the intrinsic excitability differences between DG and CA3 neurons and may participate in the low-pass filtering function of dentate granule neurons.

Molecular identity of dendritic voltage-gated sodium channels

Active invasion of the dendritic tree by action potentials (APs) generated in the axon is essential for associative synaptic plasticity and neuronal ensemble formation. In cortical pyramidal cells (PCs), this AP back-propagation is supported by dendritic voltage-gated Na+ (Nav) channels, whose molecular identity is unknown. Using a highly sensitive electron microscopic immunogold technique, we revealed the presence of the Nav1.6 subunit in hippocampal CA1 PC proximal and distal dendrites. Here, the subunit density is lower by a factor of 35 to 80 than that found in axon initial segments. A gradual decrease in Nav1.6 density along the proximodistal axis of the dendritic tree was also detected without any labeling in dendritic spines. Our results reveal the characteristic subcellular distribution of the Nav1.6 subunit, identifying this molecule as a key substrate enabling dendritic excitability.

Axon initial segment dysfunction in epilepsy

The axon initial segment (AIS) contains the site of action potential initiation and plays a major role in neuronal excitability. AIS function relies on high concentrations of different ion channels and complex regulatory mechanisms that orchestrate molecular microarchitecture. We review recent evidence that a large number of ion channels associated with epilepsy are enriched at the AIS, making it a 'hotspot' for epileptogenesis. Furthermore, we present novel data on the clustering of GABRgamma2 receptors in the AIS of cortical and hippocampal neurons in a knock in mouse model of a human genetic epilepsy. This article highlights the molecular coincidence of epilepsy mutations at the AIS and reviews pathogenic mechanisms converging at the AIS.

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.

Mechanisms and distribution of ion channels in retinal ganglion cells: using temperature as an independent variable

Trains of action potentials of rat and cat retinal ganglion cells (RGCs) were recorded intracellularly across a temperature range of 7-37 degrees C. Phase plots of the experimental impulse trains were precision fit using multicompartment simulations of anatomically reconstructed rat and cat RGCs. Action potential excitation was simulated with a "Five-channel model" [Na, K(delayed rectifier), Ca, K(A), and K(Ca-activated) channels] and the nonspace-clamped condition of the whole cell recording was exploited to determine the channels' distribution on the dendrites, soma, and proximal axon. At each temperature, optimal phase-plot fits for RGCs occurred with the same unique channel distribution. The "waveform" of the electrotonic current was found to be temperature dependent, which reflected the shape changes in the experimental action potentials and confirmed the channel distributions. The distributions are cell-type specific and adequate for soma and dendritic excitation with a safety margin. The highest Na-channel density was found on an axonal segment some 50-130 microm distal to the soma, as determined from the temperature-dependent "initial segment-somadendritic (IS-SD) break." The voltage dependence of the gating rate constants remains invariant between 7 and 23 degrees C and between 30 and 37 degrees C, but undergoes a transition between 23 and 30 degrees C. Both gating-kinetic and ion-permeability Q10s remain virtually constant between 23 and 37 degrees C (kinetic Q10s = 1.9-1.95; permeability Q10s = 1.49-1.64). The Q10s systematically increase for T <23 degrees C (kinetic Q10 = 8 at T = 8 degrees C). The Na channels were consistently "sleepy" (non-Arrhenius) for T <8 degrees C, with a loss of spiking for T <7 degrees C.

Rapid dendritic and axonal responses to neuronal insults

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system playing critical roles in basal synaptic transmission and mechanisms of learning and memory. Under normal conditions, glutamate is sequestered within synaptic vesicles (approximately 100 mM) with extracellular glutamate concentrations being limited (<1 microM), via retrieval by plasma-membrane transporters on neuronal and glial cells. In the case of central nervous system trauma, stroke, epilepsy, and in certain neurodegenerative diseases, increased concentrations of extracellular glutamate (by vesicular release, cell lysis and/or decreased glutamate transporter uptake/reversal) stimulate the overactivation of local ionotropic glutamate receptors that trigger neuronal cell death (excitotoxicity). Other natural agonists, such as domoic acid, alcohol and auto-antibodies, have also been reported to induce excitotoxicity.

Subtype-selective targeting of voltage-gated sodium channels

Voltage-gated sodium channels are key to the initiation and propagation of action potentials in electrically excitable cells. Molecular characterization has shown there to be nine functional members of the family, with a high degree of sequence homology between the channels. This homology translates into similar biophysical and pharmacological properties. Confidence in some of the channels as drug targets has been boosted by the discovery of human mutations in the genes encoding a number of them, which give rise to clinical conditions commensurate with the changes predicted from the altered channel biophysics. As a result, they have received much attention for their therapeutic potential. Sodium channels represent well-precedented drug targets as antidysrhythmics, anticonvulsants and local anaesthetics provide good clinical efficacy, driven through pharmacology at these channels. However, electrophysiological characterization of clinically useful compounds in recombinant expression systems shows them to be weak, with poor selectivity between channel types. This has led to the search for subtype-selective modulators, which offer the promise of treatments with improved clinical efficacy and better toleration. Despite developments in high-throughput electrophysiology platforms, this has proven very challenging. Structural biology is beginning to offer us a greater understanding of the three-dimensional structure of voltage-gated ion channels, bringing with it the opportunity to do real structure-based drug design in the future. This discipline is still in its infancy, but developments with the expression and purification of prokaryotic sodium channels offer the promise of structure-based drug design in the not too distant future.

The functional organization and assembly of the axon initial segment

Action potential initiation, modulation, and duration in neurons depend on a variety of Na+ and K+ channels that are highly enriched at the axon initial segment (AIS). The AIS also has high densities of cell adhesion molecules (CAMs), modulatory proteins, and a unique extracellular matrix (ECM). In contrast to other functional domains of axons (e.g. the nodes of Ranvier and axon terminals) whose development depends on the interactions with different cells (e.g. myelinating glia and postsynaptic cells), the recruitment and retention of AIS proteins is intrinsically specified through axonal cytoskeletal and scaffolding proteins. We speculate that the AIS has previously unappreciated forms of plasticity that influence neuronal excitability, and that AIS plasticity is regulated by the developmental or activity-dependent modulation of scaffolding protein levels rather than directly altering ion channel expression.

Cell-type-dependent molecular composition of the axon initial segment

The exact site of initiation and shape of action potentials vary among different neuronal types. The reason for this variability is largely unknown, but the subunit composition, density and distribution of voltage-gated sodium (Nav) and potassium (Kv) channels within the axon initial segment (AIS) are likely to play a key role. Here, we asked how heterogeneous are the density and distribution of Nav and Kv channels within the AISs of a variety of excitatory and inhibitory neurons. Most of the studied cell types expressed a high density of Nav1.6, Kv1.1, and Kv1.2 subunits in their AIS, but the Nav1.1 subunit could only be detected in GABAergic interneurons. A proximo-distal gradient in the density of these subunits was observed within the AIS of certain nerve cells but not in others. For example, a gradual increase of the Nav1.6 subunit was observed in cortical layer 2/3 and hippocampal CA1 pyramidal cell (PC) AISs, whereas its density was rather uniform in layer 5 PC AISs. The Nav1.1 subunit was distributed evenly along the AIS of short-axon cells of the main olfactory bulb but was restricted to the proximal part of the AIS in cortical and cerebellar interneurons. Our results reveal a cell type-dependent expression of sodium and potassium channel subunits with varying densities along the proximo-distal axis of the AISs. This precise arrangement is likely to contribute to the diversity of firing properties observed among central neurons.

Multiple molecular interactions determine the clustering of Caspr2 and Kv1 channels in myelinated axons

Clustering of Kv1 channels at the juxtaparanodal region (JXP) in myelinated axons depends on their association with the Caspr2/TAG-1 adhesion complex. The interaction between these channels and Caspr2 was suggested to depend on PDZ (PSD-95/Discs large/zona occludens-1) scaffolding proteins. Here, we show that at a subset of the JXP, PSD-93 colocalizes with Caspr2, K(+) channels and its related protein postsynaptic density protein-95 (PSD-95). The localization of PSD-93 and PSD-95 depends on the presence of Caspr2, as both scaffolding proteins failed to accumulate at the JXP in mice lacking either Caspr2 or TAG-1. In contrast, Caspr2 and K(+) channels still colocalized and associated in PSD-93, PSD-95 or double PSD-93/PSD-95 null mice. To directly evaluate the role of PDZ domain proteins in the function of Caspr2, we examined the ability of transgenic Caspr2 molecules lacking either their cytoplasmic domain (Caspr2dCT), or their PDZ-binding sequence (Caspr2dPDZ), to restore Kv1 channel clustering in Caspr2 null mice. We found that while Kv1 channels were distributed throughout internodes in nerves expressing Caspr2dCT, they were clustered at the JXP in axons expressing a full-length Caspr2 (Caspr2FL) or the Caspr2dPDZ transgene. Further proteomic analysis revealed that Caspr2 interacts with a distinct set of scaffolding proteins through its PDZ- and protein 4.1-binding sequences. These results demonstrate that while the molecular assembly of the JXP requires the cytoplasmic domain of Caspr2, its carboxy-terminal PDZ-binding motif is dispensable for Kv1 channel clustering. This mechanism is clearly distinct from the one operating at the axon initial segment, which requires PSD-93 for Kv1 channel clustering.