Intracellular calcium oscillations in strongly metastatic human breast and prostate cancer cells: control by voltage-gated sodium channel activity

The possible association of intracellular Ca²⁺ with metastasis in human cancer cells is poorly understood. We have studied Ca²⁺ signaling in human prostate and breast cancer cell lines of strongly versus weakly metastatic potential in a comparative approach. Intracellular free Ca²⁺ was measured using a membrane-permeant fluorescent Ca²⁺-indicator dye (Fluo-4 AM) and confocal microscopy. Spontaneous Ca²⁺ oscillations were observed in a proportion of strongly metastatic human prostate and breast cancer cells (PC-3M and MDA-MB-231, respectively). In contrast, no such oscillations were observed in weakly/non metastatic LNCaP and MCF-7 cells, although a rise in the resting Ca²⁺ level could be induced by applying a high-K⁺ solution. Various parameters of the oscillations depended on extracellular Ca²⁺ and voltage-gated Na⁺ channel activity. Treatment with either tetrodotoxin (a general blocker of voltage-gated Na⁺ channels) or ranolazine (a blocker of the persistent component of the channel current) suppressed the Ca²⁺ oscillations. It is concluded that the functional voltage-gated Na⁺ channel expression in strongly metastatic cancer cells makes a significant contribution to generation of oscillatory intracellular Ca²⁺ activity. Possible mechanisms and consequences of the Ca²⁺ oscillations are discussed.

Macrophage-sensory neuronal interaction in HIV-1 gp120-induced neurotoxicity‡

Background

Human immunodeficiency virus (HIV)-associated sensory neuropathy (SN) is the most frequent neurological complication of HIV disease. Among the probable mechanisms underlying HIV-SN are neurotoxicity induced by the HIV glycoprotein gp120 and antiretroviral therapies (ART). Since HIV-SN prevalence remains high in patients who have not been exposed to toxic ART drugs, here we focused on gp120-mediated mechanisms underlying HIV-SN.

Methods

We hypothesized that a direct gp120–sensory neurone interaction is not the cause of neurite degeneration; rather, an indirect interaction of gp120 with sensory neurones involving macrophages underlies axonal degeneration. Rat dorsal root ganglion (DRG) cultures were used to assess gp120 neurotoxicity. Rat bone marrow-derived macrophage (BMDM) cultures and qPCR array were used to assess gp120-associated gene expression changes.

Results

gp120 induced significant, but latent onset, neurite degeneration until 24 h after application. gp120–neurone interaction occurred within 1 h of application in <10% of DRG neurones, despite neurite degeneration having a global effect. Application of culture media from gp120-exposed BMDMs induced a significant reduction in DRG neurite outgrowth. Furthermore, gp120 significantly increased the expression of 25 cytokine-related genes in primary BMDMs, some of which have been implicated in other painful polyneuropathies. The C–C chemokine receptor type 5 (CCR5) antagonist, maraviroc, concentration-dependently inhibited gp120-induced tumour necrosis factor-α gene expression, indicating that these effects occurred via gp120 activation of CCR5.

Conclusions

Our findings highlight macrophages in the pathogenesis of HIV-SN and upstream modulation of macrophage response as a promising therapeutic strategy.

Identification of a receptor for neuropeptide VGF and its role in neuropathic pain

VGF (nonacronymic) is a neuropeptide precursor that plays multiple roles in regulation of energy balance, reproduction, hippocampal synaptic plasticity, and pain. Data from a number of pain models showed significant up-regulation of VGF in sensory neurons. TLQP-21, one of the VGF-derived neuropeptides, has been shown to induce a hyperalgesic response when injected subcutaneously into the hind paw of mice. However, the precise role of VGF-derived neuropeptides in neuropathic pain and the molecular identity of the receptor for VGF-derived peptides are yet to be investigated. Here we identified gC1qR, the globular heads of the C1q receptor, as the receptor for TLQP-21 using chemical cross-linking combined with mass spectrometry analysis. TLQP-21 caused an increase in intracellular Ca²⁺ levels in rat macrophages and microglia. Inoculation of TLQP-21-stimulated macrophages into rat hind paw caused mechanical hypersensitivity. The increase in intracellular Ca²⁺ levels in macrophages was attenuated by either siRNA or neutralizing antibodies against gC1qR. Furthermore, application of the gC1qR-neutralizing antibody to rats with partial sciatic nerve ligation resulted in a delayed onset of nerve injury-associated mechanical hypersensitivity. These results indicate that gC1qR is the receptor for TLQP-21 and plays an important role in chronic pain through activation of macrophages. Because direct association between TLQP-21 and gC1qR is required for activation of macrophages and causes hypersensitivity, disrupting this interaction may be a useful new approach to develop novel analgesics.

A clinically relevant rodent model of the HIV antiretroviral drug stavudine induced painful peripheral neuropathy

HIV-associated sensory neuropathy is the most frequent manifestation of HIV disease, afflicting 40-50% of patients whose HIV disease is otherwise controlled by antiretroviral therapy. It often presents with significant neuropathic pain and is consistently associated with previous exposure to nucleoside reverse transcriptase inhibitors including stavudine (d4T), which is widely used in resource-limited settings. Here we investigated complex pain-related behaviours associated with d4T treatment using ethologically relevant thigmotaxis and burrowing behaviours in adult rats. Detailed neuropathological response was also examined using neurochemistry, electron microscopy, and proteomics. After 2 intravenous injections of d4T (50 mg/kg, 4 days apart), rats developed hind paw mechanical hypersensitivity, which plateaued at 21 days after initial d4T injection, a time that these animals also had significant changes in thigmotaxis and burrowing behaviours when compared to the controls; reductions in hind paw intraepidermal nerve fibre density and CGRP/IB4 immunoreactivity in L5 spinal dorsal horn, suggesting injury to both the peripheral and central terminals of L5 dorsal root ganglion neurons; and increases in myelinated and unmyelinated axon diameters in the sural nerve, suggesting axonal swelling. However, no significant glial and inflammatory cell response to d4T treatment was observed. Sural nerve proteomics at 7 days after initial d4T injection revealed down-regulated proteins associated with mitochondrial function, highlighting distal axons vulnerability to d4T neurotoxicity. In summary, we have reported complex behavioural changes and a distinctive neuropathology in a clinically relevant rat model of d4T-induced sensory neuropathy that is suitable for further pathophysiological investigation and preclinical evaluation of novel analgesics.

Association between tetrodotoxin resistant channels and lipid rafts regulates sensory neuron excitability

Voltage-gated sodium channels (VGSCs) play a key role in the initiation and propagation of action potentials in neurons. Na_V1.8 is a tetrodotoxin (TTX) resistant VGSC expressed in nociceptors, peripheral small-diameter neurons able to detect noxious stimuli. Na_V1.8 underlies the vast majority of sodium currents during action potentials. Many studies have highlighted a key role for Na_V1.8 in inflammatory and chronic pain models. Lipid rafts are microdomains of the plasma membrane highly enriched in cholesterol and sphingolipids. Lipid rafts tune the spatial and temporal organisation of proteins and lipids on the plasma membrane. They are thought to act as platforms on the membrane where proteins and lipids can be trafficked, compartmentalised and functionally clustered. In the present study we investigated Na_V1.8 sub-cellular localisation and explored the idea that it is associated with lipid rafts in nociceptors. We found that Na_V1.8 is distributed in clusters along the axons of DRG neurons in vitro and ex vivo. We also demonstrated, by biochemical and imaging studies, that Na_V1.8 is associated with lipid rafts along the sciatic nerve ex vivo and in DRG neurons in vitro. Moreover, treatments with methyl-β-cyclodextrin (MβCD) and 7-ketocholesterol (7KC) led to the dissociation between rafts and Na_V1.8. By calcium imaging we demonstrated that the lack of association between rafts and Na_V1.8 correlated with impaired neuronal excitability, highlighted by a reduction in the number of neurons able to conduct mechanically- and chemically-evoked depolarisations. These findings reveal the sub-cellular localisation of Na_V1.8 in nociceptors and highlight the importance of the association between Na_V1.8 and lipid rafts in the control of nociceptor excitability.

In vitro and intrathecal siRNA mediated K(V)1.1 knock-down in primary sensory neurons

K(V)1.1 is a Shaker homologue K(+) channel that contributes to the juxta-paranodal membrane conductance in myelinated axons, and is blocked by fampridine (4-aminopyridine), used to treat the symptoms of multiple sclerosis. The present experiments investigate K(V)1.1 function in primary sensory neurons and A-fibres, and help define its characteristics as a drug-target using sequence specific small-interfering RNAs (siRNAs). siRNA (71nM) was used to knock-down functional expression of K(V)1.1 in sensory neurons (>25μm in apparent diameter) in culture, and was also delivered intrathecally in vivo (9.3μg). K(+) channel knock-down in sensory neurons was found to make the voltage-threshold for action potential generation significantly more negative than in control (p=0.02), led to the breakdown of accommodation and promoted spontaneous action potential firing. Exposure to dendrotoxin-K (DTX-K, 10-100nM) also selectively abolished K(+) currents at negative potentials and made voltage-threshold more negative, consistent with K(V)1.1 controlling excitability close to the nominal resting potential of the neuron cell body, near -60mV. Introduction of one working siRNA sequence into the intrathecal space in vivo was associated with a small increase in the amplitude of the depolarising after-potential in sacral spinal roots (p<0.02), suggesting a reduction in the number of working K(+) channels in internodal axon membrane. Our study provides evidence that K(V)1.1 contributes to the control of peripheral sensory nerve excitability, and suggests that its characteristics as a putative drug target can be assessed by siRNA transfection in primary sensory neurons in vitro and in vivo.

Building Excitable Membranes

Multiple plasma membrane proteins such as ion transporters and ion channels are involved in electrogenesis by setting resting membrane potentials and triggering/propagating action potentials. Recent findings strongly suggest that some of these membrane proteins are selectively transported into membrane microdomains termed lipid rafts. There appear to be multiple mechanisms for the specific protein translocation to lipid rafts, and many of these proteins exhibit distinct properties when inserted into the raft microdomains. Here the authors review the plasma membrane ion channels specifically localized at membrane lipid rafts in neurons. The mechanisms to selectively translocate these molecules to the lipid rafts and the consequences of the trafficking are also discussed.

The trafficking of Na(V)1.8

The α-subunit of tetrodotoxin-resistant voltage-gated sodium channel Na(V)1.8 is selectively expressed in sensory neurons. It has been reported that Na(V)1.8 is involved in the transmission of nociceptive information from sensory neurons to the central nervous system in nociceptive [1] and neuropathic [24] pain conditions. Thus Na(V)1.8 has been a promising target to treat chronic pain. Here we discuss the recent advances in the study of trafficking mechanism of Na(V)1.8. These pieces of information are particularly important as such trafficking machinery could be new targets for painkillers.

Increased basal insulin secretion in Pdzd2-deficient mice

Expression of the multi-PDZ protein Pdzd2 (PDZ domain-containing protein 2) is enriched in pancreatic islet beta cells, but not in exocrine or alpha cells, suggesting a role for Pdzd2 in the regulation of pancreatic beta-cell function. To explore the in vivo function of Pdzd2, Pdzd2-deficient mice were generated. Homozygous Pdzd2 mutant mice were viable and their gross morphology appeared normal. Interestingly, Pdzd2-deficient mice showed enhanced glucose tolerance in intraperitoneal glucose tolerance tests and their plasma insulin levels indicated increased basal insulin secretion after fasting. Moreover, insulin release from mutant pancreatic islets was found to be twofold higher than from normal islets. To verify the functional defect in vitro, Pdzd2 was depleted in INS-1E cells using two siRNA duplexes. Pdzd2-depleted INS-1E cells also displayed increased insulin secretion at low concentrations of glucose. Our results provide the first evidence that Pdzd2 is required for normal regulation of basal insulin secretion.

A multi PDZ-domain protein Pdzd2 contributes to functional expression of sensory neuron-specific sodium channel Na(V)1.8

The voltage-gated sodium channel Na_V1.8 is expressed exclusively in nociceptive sensory neurons and plays an important role in pain pathways. Na_V1.8 cannot be functionally expressed in non-neuronal cells even in the presence of β-subunits. We have previously identified Pdzd2, a multi PDZ-domain protein, as a potential interactor for Na_V1.8. Here we report that Pdzd2 binds directly to the intracellular loops of Na_V1.8 and Na_V1.7. The endogenous Na_V1.8 current in sensory neurons is inhibited by antisense- and siRNA-mediated downregulation of Pdzd2. However, no marked change in pain behaviours is observed in Pdzd2-decificent mice. This may be due to compensatory upregulation of p11, another regulatory factor for Na_V1.8, in dorsal root ganglia of Pdzd2-deficient mice. These findings reveal that Pdzd2 and p11 play collaborative roles in regulation of Na_V1.8 expression in sensory neurons.

Proteomic profiling of neuromas reveals alterations in protein composition and local protein synthesis in hyper-excitable nerves

Neuropathic pain may arise following peripheral nerve injury though the molecular mechanisms associated with this are unclear. We used proteomic profiling to examine changes in protein expression associated with the formation of hyper-excitable neuromas derived from rodent saphenous nerves. A two-dimensional difference gel electrophoresis (2D-DIGE) profiling strategy was employed to examine protein expression changes between developing neuromas and normal nerves in whole tissue lysates. We found around 200 proteins which displayed a >1.75-fold change in expression between neuroma and normal nerve and identified 55 of these proteins using mass spectrometry. We also used immunoblotting to examine the expression of low-abundance ion channels Nav1.3, Nav1.8 and calcium channel alpha2delta-1 subunit in this model, since they have previously been implicated in neuronal hyperexcitability associated with neuropathic pain. Finally, S35methionine in vitro labelling of neuroma and control samples was used to demonstrate local protein synthesis of neuron-specific genes. A number of cytoskeletal proteins, enzymes and proteins associated with oxidative stress were up-regulated in neuromas, whilst overall levels of voltage-gated ion channel proteins were unaffected. We conclude that altered mRNA levels reported in the somata of damaged DRG neurons do not necessarily reflect levels of altered proteins in hyper-excitable damaged nerve endings. An altered repertoire of protein expression, local protein synthesis and topological re-arrangements of ion channels may all play important roles in neuroma hyper-excitability.

Pain signalling pathways: from cytokines to ion channels

Pain is the major reason patients seek medical care. The treatment of pain, particularly chronic pain associated with cancer and damage to the nervous system, is at present inadequate. Lack of effective analgesics is partly due to the fact that pain signalling mechanisms are still not fully understood. Over the recent years, many channels, receptors, and regulatory proteins involved in pain pathways have bee identified, and novel pain signalling mechanisms and pathways at peripheral and spinal levels have been discovered. It is anticipated that increased understanding of the molecular mechanisms of pain would provide a hope for the future development of effective pain killers. This review examines the currently available information on the molecular aspects of pain signalling pathways, and discusses novel and promising therapeutic targets for the treatment of pain in humans.

Deletion of annexin 2 light chain p11 in nociceptors causes deficits in somatosensory coding and pain behavior

The S100 family protein p11 (S100A10, annexin 2 light chain) is involved in the trafficking of the voltage-gated sodium channel Na(V)1.8, TWIK-related acid-sensitive K+ channel (TASK-1), the ligand-gated ion channels acid-sensing ion channel 1a (ASIC1a) and transient receptor potential vanilloid 5/6 (TRPV5/V6), as well as 5-hydroxytryptamine receptor 1B (5-HT1B), a G-protein-coupled receptor. To evaluate the role of p11 in peripheral pain pathways, we generated a loxP-flanked (floxed) p11 mouse and used the Cre-loxP recombinase system to delete p11 exclusively from nociceptive primary sensory neurons in mice. p11-null neurons showed deficits in the expression of Na(V)1.8, but not of annexin 2. Damage-sensing primary neurons from these animals show a reduced tetrodotoxin-resistant sodium current density, consistent with a loss of membrane-associated Na(V)1.8. Noxious coding in wide-dynamic-range neurons in the dorsal horn was markedly compromised. Acute pain behavior was attenuated in certain models, but no deficits in inflammatory pain were observed. A significant deficit in neuropathic pain behavior was also apparent in the conditional-null mice. These results confirm an important role for p11 in nociceptor function.

Intense isolectin-B4 binding in rat dorsal root ganglion neurons distinguishes C-fiber nociceptors with broad action potentials and high Nav1.9 expression

Binding to isolectin-B4 (IB4) and expression of tyrosine kinase A (trkA) (the high-affinity NGF receptor) have been used to define two different subgroups of nociceptive small dorsal root ganglion (DRG) neurons. We previously showed that only nociceptors have high trkA levels. However, information about sensory and electrophysiological properties in vivo of single identified IB4-binding neurons, and about their trkA expression levels, is lacking. IB4-positive (IB4+) and small dark neurons had similar size distributions. We examined IB4-binding levels in >120 dye-injected DRG neurons with sensory and electrophysiological properties recorded in vivo. Relative immunointensities for trkA and two TTX-resistant sodium channels (Nav1.8 and Nav1.9) were also measured in these neurons. IB4+ neurons were classified as strongly or weakly IB4+. All strongly IB4+ neurons were C-nociceptor type (C-fiber nociceptive or unresponsive). Of 32 C-nociceptor-type neurons examined, approximately 50% were strongly IB4+, approximately 20% were weakly IB4+ and approximately 30% were IB4-. Adelta low-threshold mechanoreceptive (LTM) neurons were weakly IB4+ or IB4-. All 33 A-fiber nociceptors and all 44 Aalpha/beta-LTM neurons examined were IB4-. IB4+ compared with IB4- C-nociceptor-type neurons had longer somatic action potential durations and rise times, slower conduction velocities, more negative membrane potentials, and greater immunointensities for Nav1.9 but not Nav1.8. Immunointensities of IB4 binding in C-neurons were positively correlated with those of Nav1.9 but not Nav1.8. Of 23 C-neurons tested for both trkA and IB4, approximately 35% were trkA+/IB4+ but with negatively correlated immunointensities; 26% were IB4+/trkA-, and 35% were IB4-/trkA+. We conclude that strongly IB4+ DRG neurons are exclusively C-nociceptor type and that high Nav1.9 expression may contribute to their distinct membrane properties.

Annexin II light chain p11 promotes functional expression of acid-sensing ion channel ASIC1a

Acid-sensing ion channels (ASICs) have been implicated in a wide variety of physiological functions. We have used a rat dorsal root ganglion cDNA library in a yeast two-hybrid assay to identify sensory neuron proteins that interact with ASICs. We found that annexin II light chain p11 physically interacts with the N terminus of ASIC1a, but not other ASIC isoforms. Immunoprecipitation studies confirmed an interaction between p11 and ASIC1 in rat dorsal root ganglion neurons in vivo. Coexpression of p11 and ASIC1a in CHO-K1 cells led to a 2-fold increase in expression of the ion channel at the cell membrane as determined by membrane-associated immunoreactivity and cell-surface biotinylation. Consistent with these findings, peak ASIC1a currents in transfected CHO-K1 cells were up-regulated 2-fold in the presence of p11, whereas ASIC3-mediated currents were unaffected by p11 expression. Neither the pH dependence of activation nor the rates of desensitization were altered by p11, suggesting that its primary role in regulating ASIC1a activity is to enhance cell-surface expression of ASIC1a. These data demonstrate that p11, already known to traffic members of the voltage-gated sodium and potassium channel families as well as transient receptor potential and chloride channels, also plays a selective role in enhancing ASIC1a functional expression.

trkA is expressed in nociceptive neurons and influences electrophysiological properties via Nav1.8 expression in rapidly conducting nociceptors

To test the hypothesis that trkA (the high-affinity NGF receptor) is selectively expressed in nociceptive dorsal root ganglion (DRG) neurons, we examined the intensity of trkA immunoreactivity in single dye-injected rat DRG neurons, the sensory receptor properties of which were identified in vivo with mechanical and thermal stimuli. We provide the first evidence in single identified neurons that strong trkA expression in DRGs is restricted to nociceptive neurons, probably accounting for the profound influence of NGF on these neurons. Furthermore, we demonstrate that trkA expression is as high in rapidly conducting (Aalpha/beta) as in more slowly conducting (Adelta and C) nociceptors. All Aalpha/beta low-threshold mechanoreceptors (LTMs) are trkA negative, although weak but detectable trkA is present in some C and Adelta LTMs. NGF can influence electrophysiological properties of DRG neurons, probably by binding to trkA. We found positive correlations for single identified Aalpha/beta (but not C or Adelta) nociceptors between trkA immunocytochemical intensity and electrophysiological properties typical of nociceptors, namely long action potential and afterhyperpolarization durations and large action potential amplitudes. Furthermore, for Aalpha/beta (notCorAdelta) nociceptors, trkA intensity is inversely correlated with conduction velocity. Similar relationships, again only in Aalpha/beta nociceptors, between electrophysiological properties and trkA expression exist for sodium channel Nav1.8 but not Nav1.9 immunoreactivities. These findings suggest that in Aalpha/beta nociceptors, influences of NGF on expression levels of Nav1.8 are related to, and perhaps limited by, expression levels of trkA. This view is supported by a positive correlation between immuno-intensities of trkA and Nav1.8 in A-fiber, but not C-fiber, nociceptors.

Identification of binding domains in the sodium channel Na(V)1.8 intracellular N-terminal region and annexin II light chain p11

The interaction of p11 (annexin II light chain) with the N-terminal domain of Na(V)1.8, a tetrodotoxin-resistant sodium channel, is essential for the functional expression of the channel. Here we show that p11 binds to Na(V)1.8 but not to sodium channel isoforms Na(V)1.2, 1.5, 1.7 or Na(V)1.9. The binding of amino acids 74-103 of Na(V)1.8 to p11 residues 33-78 occurs in a random coiled region flanked by two EF hand motifs whose crystal structure has been established. As Na(V)1.8 channel expression is associated with pain pathways, drugs that disrupt the Na(V)1.8-p11 interaction and down-regulate channel expression may have analgesic activity.

Voltage-gated sodium channels and pain pathways

Acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. The peripheral input that drives pain perception thus depends on the presence of functional voltage-gated sodium channels. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. A combination of antisense and knock-out studies support a specialized role for these sodium channels in pain pathways, and pharmacological studies with conotoxins suggest that isotype-specific antagonists should be feasible. Taken together, these data suggest that isotype-specific sodium channel blockers could be useful analgesics.

Ion channel activities implicated in pathological pain

Altered expression of voltage-gated sodium, calcium and potassium channels has been associated with neuropathic pain conditions. In addition, roles for the ligand-gated P2X3 and NMDA receptors, as well as pacemaker HCN channels have also been invoked in the pathogenesis of neuropathic pain. In this chapter, evidence of an important role for post-translational regulation of Nav1.9 in setting pain thresholds is presented. Despite the importance of tactile allodynia and mechanical hyperalgesia in chronic pain, we remain ignorant of the molecular nature of mechanosensors present in sensory neurons. A number of candidate mechanosensor genes, identified because of their structural similarity with mechanosensors in Caenorbabditis elegans and Drosophila melanogaster have been identified. Acid-sensing ion channels (ASICs) are structurally related to putative mechanosensors in C. elegans, whilst transient receptor potential channels (TRPs) have been implicated in mechanosensation in the Drosophila acoustic system. Evidence against a role for ASICs as primary transducers of mechanosensation is provided here, and recent evidence implicating TRP channels is reviewed. Finally, the use of sensory neuron-specific gene deletion approaches to unravel the significance of individual ion channels in the regulation of sensory neuron excitability and the induction of pain will be described.

The TTX-resistant sodium channel Nav1.8 (SNS/PN3): expression and correlation with membrane properties in rat nociceptive primary afferent neurons

We have examined the distribution of the sensory neuron-specific Na+ channel Nav1.8 (SNS/PN3) in nociceptive and non-nociceptive dorsal root ganglion (DRG) neurons and whether its distribution is related to neuronal membrane properties. Nav1.8-like immunoreactivity (Nav1.8-LI) was examined with an affinity purified polyclonal antiserum (SNS11) in rat DRG neurons that were classified according to sensory receptive properties and by conduction velocity (CV) as C-, Adelta- or Aalpha/beta. A significantly higher proportion of nociceptive than low threshold mechanoreceptive (LTM) neurons showed Nav1.8-LI, and nociceptive neurons had significantly more intense immunoreactivity in their somata than LTM neurons. Results showed that 89, 93 and 60% of C-, Adelta- and Aalpha/beta-fibre nociceptive units respectively and 88% of C-unresponsive units were positive. C-unresponsive units had electrical membrane properties similar to C-nociceptors and were considered to be nociceptive-type neurons. Weak positive Nav1.8-LI was also present in some LTM units including a C LTM, all Adelta LTM units (D hair), about 10% of cutaneous LTM Aalpha/beta-units, but no muscle spindle afferent units. Nav1.8-LI intensity was negatively correlated with soma size (all neurons) and with dorsal root CVs in A- but not C-fibre neurons. Nav1.8-LI intensity was positively correlated with action potential (AP) duration (both rise and fall time) in A-fibre neurons and with AP rise time only in positive C-fibre neurons. It was also positively correlated with AP overshoot in positive neurons. Thus high levels of Nav1.8 protein may contribute to the longer AP durations (especially in A-fibre neurons) and larger AP overshoots that are typical of nociceptors.

Sensory neuron proteins interact with the intracellular domains of sodium channel NaV1.8

Voltage-gated sodium channels initiate and propagate action potentials in excitable cells. The tetrodotoxin-resistant Na(+) channel (Na(V)1.8/SNS) is expressed in damage-sensing neurons (nociceptors) and plays an important role in pain pathways. Expression of high levels of functional Na(V)1.8 in heterologous cells has proved problematic, even in the presence of known sodium channel accessory beta-subunits. This suggests that other regulatory proteins are required for normal levels of Na(V)1.8 expression. Here we report the use of a yeast two-hybrid system and a rat dorsal root ganglion cDNA library to identify 28 different clones encoding proteins which interact with intracellular domains of Na(V)1.8. Many clones are expressed at high levels in small diameter DRG neurons as judged by in situ hybridization. Interacting proteins include cytoplasmic elements and linker proteins (e.g. beta-actin and moesin), enzymes (e.g. inositol polyphosphate 5-phosphatase and TAO2 thousand and one protein kinase), channels and membrane-associated proteins (voltage-dependent anion channel VDAC3V and tetraspanin), as well as motor proteins (dynein intermediate and light chain) and transcripts encoding previously undescribed proteins. Immunoprecipitation (pull-down) assays confirm that some of the proteins interact with, and may hence regulate, Na(V)1.8 in vivo.

Annexin II light chain regulates sensory neuron-specific sodium channel expression

The tetrodotoxin-resistant sodium channel Na(V)1.8/SNS is expressed exclusively in sensory neurons and appears to have an important role in pain pathways. Unlike other sodium channels, Na(V)1.8 is poorly expressed in cell lines even in the presence of accessory beta-subunits. Here we identify annexin II light chain (p11) as a regulatory factor that facilitates the expression of Na(V)1.8. p11 binds directly to the amino terminus of Na(V)1.8 and promotes the translocation of Na(V)1.8 to the plasma membrane, producing functional channels. The endogenous Na(V)1.8 current in sensory neurons is inhibited by antisense downregulation of p11 expression. Because direct association with p11 is required for functional expression of Na(V)1.8, disrupting this interaction may be a useful new approach to downregulating Na(V)1.8 and effecting analgesia.

Sodium channels in primary sensory neurons: relationship to pain states

Electrophysiological studies of dorsal root ganglion (DRG) neurons, and the results of PCR, Northern blot and in situ hybridization analyses have demonstrated the molecular diversity of Na+ channels that operate in sensory neurons. Several subtypes of alpha-subunit have been detected in DRG neurons and transcripts encoding all three beta-subunits are also present. Interestingly, one alpha subunit, Na(v)1.8, is selectively expressed in C-fibre and Adelta fibre associated sensory neurons that are predominantly involved in damage sensing. Another channel, Na(v).3, is selectively up regulated in a variety of models of neuropathic pain. In this review we focus on Na+ channels that are selectively expressed in DRG neurons as potential analgesic drug targets. In the absence of subtype specific inhibitors, the production of null mutant mice provides useful information on the specialized functions of particular Na+ channels. A refinement of this approach is to delete Na+ channel genes flanked by lox-P sites in the sensory ganglia of adult animals, using viruses to deliver the bacteriophage Cre recombinase enzyme.

Potent analgesic effects of GDNF in neuropathic pain states

Neuropathic pain arises as a debilitating consequence of nerve injury. The etiology of such pain is poorly understood, and existing treatment is largely ineffective. We demonstrate here that glial cell line-derived neurotrophic factor (GDNF) both prevented and reversed sensory abnormalities that developed in neuropathic pain models, without affecting pain-related behavior in normal animals. GDNF reduces ectopic discharges within sensory neurons after nerve injury. This may arise as a consequence of the reversal by GDNF of the injury-induced plasticity of several sodium channel subunits. Together these findings provide a rational basis for the use of GDNF as a therapeutic treatment for neuropathic pain states.

The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways

Many damage-sensing neurons express tetrodotoxin (TTX)-resistant voltage-gated sodium channels. Here we examined the role of the sensory-neuron-specific (SNS) TTX-resistant sodium channel alpha subunit in nociception and pain by constructing sns-null mutant mice. These mice expressed only TTX-sensitive sodium currents on step depolarizations from normal resting potentials, showing that all slow TTX-resistant currents are encoded by the sns gene. Null mutants were viable, fertile and apparently normal, although lowered thresholds of electrical activation of C-fibers and increased current densities of TTX-sensitive channels demonstrated compensatory upregulation of TTX-sensitive currents in sensory neurons. Behavioral studies demonstrated a pronounced analgesia to noxious mechanical stimuli, small deficits in noxious thermoreception and delayed development of inflammatory hyperalgesia. These data show that SNS is involved in pain pathways and suggest that blockade of SNS expression or function may produce analgesia without side effects.

cAMP-dependent phosphorylation of the tetrodotoxin-resistant voltage-dependent sodium channel SNS

1. Protein kinase A (PKA) modulation of tetrodotoxin-resistant (TTX-r) voltage-gated sodium channels may underly the hyperalgesic responses of mammalian sensory neurones. We have therefore examined PKA phosphorylation of the cloned alpha-subunit of the rat sensory neurone-specific TTX-r channel SNS. Phosphorylation of SNS was compared with that of a mutant channel, SNS(SA), in which all five PKA consensus sites (RXXS) within the intracellular I-II loop had been eliminated by site-directed mutagenesis (serine to alanine). 2. In vitro PKA phosphorylation and tryptic peptide mapping of SNS and mutant SNS(SA) I-II loops expressed as glutathione-S-transferase (GST) fusion proteins confirmed that the five mutated serines were the major PKA substrates within the SNS I-II loop. 3. SNS and SNS(SA) channels were transiently expressed in COS-7 cells and their electrophysiological properties compared. In wild-type SNS channels, forskolin and 8-bromo cAMP produced effects consistent with PKA phosphorylation. Mutant SNS(SA) currents, however, were not significantly affected by either agent. Thus, elimination of the I-II loop PKA consensus sites caused a marked reduction in PKA modulation of wild-type channels. 4. Under control conditions, the voltage dependence of activation of SNS(SA) current was shifted to depolarized potentials compared with SNS. This was associated with a slowing of SNS(SA) current inactivation at hyperpolarized potentials and suggested a tonic PKA phosphorylation of wild-type channels under basal conditions.5. We conclude that the major substrates involved in functional PKA modulation of the SNS channel are located within the intracellular I-II loop.

Trans-splicing of a voltage-gated sodium channel is regulated by nerve growth factor

Mammalian sensory neurons express a voltage-gated sodium channel named SNS. Here we report the identification of an SNS transcript (SNS-A) that contains an exact repeat of exons 12, 13 and 14 encoding a partial repeat of domain II. Because the exons 12-14 are present in single copies in genomic DNA, the SNS-A transcript must arise by trans-splicing. Nerve growth factor, which regulates pain thresholds, and the functional expression of voltage-gated sodium channels increases the levels of the SNS-A transcript several-fold both in vivo and in vitro as measured by RNase protection methods, as well as RT-PCR. These data demonstrate a novel regulatory role for the nerve growth factor and are the first example of trans-splicing in the vertebrate nervous system.

Regulation of expression of the sensory neuron-specific sodium channel SNS in inflammatory and neuropathic pain

Increased voltage-gated sodium channel activity may contribute to the hyperexcitability of sensory neurons in inflammatory and neuropathic pain states. We examined the levels of the transcript encoding the tetrodotoxin-resistant sodium channel SNS in dorsal root ganglion neurons in a range of inflammatory and neuropathic pain models in the rat. Local Freund's adjuvant or systemic nerve growth factor-induced inflammation did not substantially alter the total levels of SNS mRNA. When NGF-treated adult rat DRG neurons in vitro were compared with NGF-depleted control neurons, SNS total mRNA levels and the levels of membrane-associated immunoreactive SNS showed a small increase (17 and 25%, respectively), while CGRP levels increased fourfold. SNS expression is thus little dependent on NGF even though SNS transcript levels dropped by more than 60% 7-14 days after axotomy. In the streptozotocin diabetic rat SNS levels fell 25%, while in several manipulations of the L5/6 tight nerve ligation rat neuropathic pain model, SNS levels fell 40-80% in rat strains that are either susceptible or relatively resistant to the development of allodynia. Increased expression of SNS mRNA is thus unlikely to underlie sensory neuron hyperexcitability associated with inflammation, while lowered SNS transcript levels are associated with peripheral nerve damage.

A single serine residue confers tetrodotoxin insensitivity on the rat sensory-neuron-specific sodium channel SNS

Sensory neurons express a sodium channel (SNS) that is highly resistant to block by tetrodotoxin (IC50 = 60 microM). SNS is 65% homologous to the cardiac sodium channel, in which a single hydrophilic residue in the SS2 segment is critical for tetrodotoxin resistance. By site-directed mutagenesis, we have substituted phenylalanine for serine at the equivalent position in SNS: this mutated (S356F) SNS channel is functionally similar to wild-type SNS when expressed in Xenopus oocytes, but is potently blocked by tetrodotoxin and saxitoxin with IC50s of 2.8 nM and 8.2 nM, respectively. These data provide clues to the rational design of selective blockers of SNS with potential as analgesic drugs.

Tissue-specific methylation occurs in the essential promoter element of the tyrosine hydroxylase gene

Expression of tyrosine hydroxylase (TH) is regulated in a tissue-specific manner by multiple mechanisms. In catecholaminergic cells, the expression of TH-mRNA is up-regulated by forskolin (FK) and is suppressed by retinoic acid (RA). We have previously provided evidence that, in N-18 cells, the expression of TH-mRNA is suppressed by DNA methylation of the TH gene itself. In the present study, using a catecholaminergic cell line, N1E-115, we performed deletional and mutational analyses on the 5'-flanking region of the mouse TH gene. The results indicate that a cAMP response element (CRE) mediates constitutive transcription of the TH gene, as well as responsiveness to FK and RA. Using bisulfite sequencing methods, we analyzed the methylation status of the TH gene 5'-flanking region in various cell lines and rat tissues. We found that three cytosine residues in the domain surrounding the CRE of the TH gene promoter were specifically methylated in N-18 cells and TH non-expressing rat tissues. In contrast, these cytosines were undermethylated in TH expressing cell lines and tissues. The inverse correlation between the frequency of cytosine methylation at these specific sites and the levels of TH expression supports a role for DNA methylation in the regulation of tissue-specific gene expression.

Induction of cholinergic and adrenergic differentiation in N-18 cells by differentiation agents and DNA demethylating agents

Effects of various differentiating agents and DNA demethylating agents on the expression of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH), marker enzymes for cholinergic and adrenergic differentiation, respectively, were examined in N-18 neuroblastoma cells. Retinoic acid (RA) and a medium conditioned over C6-glioma cells (GCM), which have been shown to enhance the ChAT activity of PC12 cells, NG108-15 cells and fetal rat brain cells, did not induce ChAT activity of N-18 cells. Treatment of the cells with the DNA demethylating agents alone also did not affect ChAT activity. But after pretreatment of the cells with the DNA demethylating agents, ChAT activity of N-18 cells was greatly increased by either RA or GCM. TH activity of N-18 cells was enhanced by forskolin, an activator of adenylate cyclase. The pretreatment of the cells with the DNA demethylating agents greatly enhanced the induction of TH activity by forskolin. Levels of ChAT and TH messenger RNA were altered in accordance with changes in ChAT and TH activities. Possible mechanisms of the actions of the demethylating agents on cholinergic and adrenergic differentiation are discussed.

Marginal closure of non-gamma2 amalgam

In order to investigate the effect of burnishing on the marginal closure of non-gamma2 amalgam restorations, a single-composition high-copper alloy (Indiloy), a conventional lathe-cut alloy (Lunargent Alloy), and a conventional spherical alloy (Shofu Spherical) were each mixed with mercury and filled in transparent plastic cavities. Half of the specimens were burnished along the cavity margins immediately after packing and again after carving. The remainder was unburnished. A dye was sprayed on their occlusal surfaces after 24 hr and leakage indicated by the dye penetration was observed through the plastic walls, and following facts were found: 1. Burnishing markedly decreased the leakage of all type alloy amalgams. 2. When burnished, the marginal leakage with the high-copper alloy was remarkably less than with the conventional spherical alloy but slightly more than that with the lathe-cut alloy. 3. The difference in the marginal leakage and in the effect of burnishing among the three amalgams was apparently related to their setting dimensional change curves.

Relationship between hardness, discoloration, and microbial invasion in carious dentin

SYNOPSIS IN INTERLINGUA

LE RELATION INTER LE DURITATE, LE DISCOLORATION, E LE INVASION MICROBIAL IN DENTINA CARIOSE.-Frescamente extrahite dentes con carie esseva dissecate verticalmente. In un del partes le superficie sectional esseva examinate pro duritate secundo Knoop e discoloration. Sectiones histologic del altere parte esseva usate pro le detection de evidentia microbiotic. Le resultatos esseva le sequentes: In dentina cariose, mollification precedeva in omne casos le discoloration, e invasion microbial esseva semper le tertie stadio. Le duritate de dentina in le fronte discolorate o microbial esseva inferior in casos acute e superior in casos chronic. Le distantia inter ille frontes e le fronte de mollification esseva plus grande in casos acute e plus micre in casos chronic.