Stable Expression and Characterization of Human PN1 and PN3 Sodium Channels

Voltage-gated sodium channels: from roles and mechanisms in the metastatic cell behavior to clinical potential as therapeutic targets

During the second half of the last century, the prevalent knowledge recognized the voltage-gated sodium channels (VGSCs) as the proteins responsible for the generation and propagation of action potentials in excitable cells. However, over the last 25 years, new non-canonical roles of VGSCs in cancer hallmarks have been uncovered. Their dysregulated expression and activity have been associated with aggressive features and cancer progression towards metastatic stages, suggesting the potential use of VGSCs as cancer markers and prognostic factors. Recent work has elicited essential information about the signalling pathways modulated by these channels: coupling membrane activity to transcriptional regulation pathways, intracellular and extracellular pH regulation, invadopodia maturation, and proteolytic activity. In a promising scenario, the inhibition of VGSCs with FDA-approved drugs as well as with new synthetic compounds, reduces cancer cell invasion in vitro and cancer progression in vivo. The purpose of this review is to present an update regarding recent advances and ongoing efforts to have a better understanding of molecular and cellular mechanisms on the involvement of both pore-forming α and auxiliary β subunits of VGSCs in the metastatic processes, with the aim at proposing VGSCs as new oncological markers and targets for anticancer treatments.

Characterization of Endogenous Sodium Channels in the ND7-23 Neuroblastoma Cell Line: Implications for Use as a Heterologous Ion Channel Expression System Suitable for Automated Patch Clamp Screening

The rodent neuroblastoma cell line, ND7-23, is used to express voltage-dependent sodium (Nav) and other neuronal ion channels resistant to heterologous expression in Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells. Their advantage is that they provide endogenous factors and signaling pathways to promote ion channel peptide folding, expression, and function at the cell surface and are also amenable to automated patch clamping. However, ND7-23 cells exhibit endogenous tetrodotoxin (TTX)-sensitive Nav currents, and molecular profiling has revealed the presence of Nav1.2, Nav1.3, Nav1.6, and Nav1.7 transcripts, but no study has determined which subtypes contribute to functional channels at the cell surface. We profiled the repertoire of functional Nav channels endogenously expressed in ND7-23 cells using the QPatch automated patch clamp platform and selective toxins and small molecules. The potency and subtype selectivity of the ligands (Icagen compound 68 from patent US-20060025415-A1-20060202, 4,9 anhydro TTX, and Protoxin-II) were established in human Nav1.3, Nav1.6, and Nav1.7 channel cell lines before application of selective concentrations to ND7-23 cells. Our data confirm previous studies that >97% of macroscopic Nav current in ND7-23 cells is carried by TTX-sensitive channels (300 nM TTX) and that Nav1.7 is the predominant channel contributing to this response (65% of peak inward current), followed by Nav1.6 (∼20%) and negligible Nav1.3 currents (∼2%). In addition, our data are the first to assess the Nav1.6 potency (50% inhibitory concentration [IC₅₀] of 33 nM) and selectivity (50-fold over Nav1.7) of 4,9 anhydro TTX in human Nav channels expressed in mammalian cells, confirming previous studies of rodent Nav channels expressed in oocytes and HEK cells.

Development and validation of a thallium flux-based functional assay for the sodium channel NaV1.7 and its utility for lead discovery and compound profiling

Ion channels are critical for life, and they are targets of numerous drugs. The sequencing of the human genome has revealed the existence of hundreds of different ion channel subunits capable of forming thousands of ion channels. In the face of this diversity, we only have a few selective small-molecule tools to aid in our understanding of the role specific ion channels in physiology which may in turn help illuminate their therapeutic potential. Although the advent of automated electrophysiology has increased the rate at which we can screen for and characterize ion channel modulators, the technique's high per-measurement cost and moderate throughput compared to other high-throughput screening approaches limit its utility for large-scale high-throughput screening. Therefore, lower cost, more rapid techniques are needed. While ion channel types capable of fluxing calcium are well-served by low cost, very high-throughput fluorescence-based assays, other channel types such as sodium channels remain underserved by present functional assay techniques. In order to address this shortcoming, we have developed a thallium flux-based assay for sodium channels using the NaV1.7 channel as a model target. We show that the assay is able to rapidly and cost-effectively identify NaV1.7 inhibitors thus providing a new method useful for the discovery and profiling of sodium channel modulators.

Functional Na V 1.8 Channels in Intracardiac Neurons

Rationale:

The SCN10A gene encodes the neuronal sodium channel isoform Na V 1.8. Several recent genome-wide association studies have linked SCN10A to PR interval and QRS duration, strongly suggesting an as-yet unknown role for Na V 1.8 in cardiac electrophysiology.

Objective:

To demonstrate the functional presence of SCN10A /Nav1.8 in intracardiac neurons of the mouse heart.

Methods and Results:

Immunohistochemistry on mouse tissue sections showed intense Na V 1.8 labeling in dorsal root ganglia and intracardiac ganglia and only modest Na V 1.8 expression within the myocardium. Immunocytochemistry further revealed substantial Na V 1.8 staining in isolated neurons from murine intracardiac ganglia but no Na V 1.8 expression in isolated ventricular myocytes. Patch-clamp studies demonstrated that the Na V 1.8 blocker A-803467 (0.5–2 μmol/L) had no effect on either mean sodium current (I Na ) density or I Na gating kinetics in isolated myocytes but significantly reduced I Na density in intracardiac neurons. Furthermore, A-803467 accelerated the slow component of current decay and shifted voltage dependence of inactivation toward more negative voltages, as expected for blockade of Na V 1.8-based I Na . In line with these findings, A-803467 did not affect cardiomyocyte action potential upstroke velocity but markedly reduced action potential firing frequency in intracardiac neurons, confirming a functional role for Na V 1.8 in cardiac neural activity.

Conclusions:

Our findings demonstrate the functional presence of SCN10A /Na V 1.8 in intracardiac neurons, indicating a novel role for this neuronal sodium channel in regulation of cardiac electric activity.

Structural determinants of drugs acting on the Nav1.8 channel

The aim of this work is to study the role of pore residues on drug binding in the Na(V)1.8 channel. Alanine mutations were made in the S6 segments, chosen on the basis of their roles in other Na(V) subtypes; whole cell patch clamp recordings were made from mammalian ND7/23 cells. Mutations of some residues caused shifts in voltage dependence of activation and inactivation, and gave faster time course of inactivation, indicating that the residues mutated play important roles in both activation and inactivation in the Na(V)1.8 channel. The resting and inactivated state affinities of tetracaine for the channel were reduced by mutations I381A, F1710A, and Y1717A (for the latter only inactivated state affinity was measured), and by mutation F1710A for the Na(V)1.8-selective compound A-803467, showing the involvement of these residues for each compound, respectively. For both compounds, mutation L1410A caused the unexpected appearance of a complete resting block even at extremely low concentrations. Resting block of native channels by compound A-803467 could be partially removed ("disinhibition") by repetitive stimulation or by a test pulse after recovery from inactivation; the magnitude of the latter effect was increased for all the mutants studied. Tetracaine did not show this effect for native channels, but disinhibition was seen particularly for mutants L1410A and F1710A. The data suggest differing, but partially overlapping, areas of binding of A-803467 and tetracaine. Docking of the ligands into a three-dimensional model of the Na(V)1.8 channel gave interesting insight as to how the ligands may interact with pore residues.

Lidocaine Promotes the Trafficking and Functional Expression of Nav1.8 Sodium Channels in Mammalian Cells

Nociceptive neurons of the dorsal root ganglion (DRG) express a combination of rapidly gating TTX-sensitive and slowly gating TTX-resistant Na currents, and the channels that produce these currents have been cloned. The Nav1.7 and Nav1.8 channels encode for the rapidly inactivating TTX-sensitive and slowly inactivating TTX-resistant Na currents, respectively. Although the Nav1.7 channel expresses well in cultured mammalian cell lines, attempts to express the Nav1.8 channel using similar approaches has been met with limited success. The inability to heterologously express Nav1.8 has hampered detailed characterization of the biophysical properties and pharmacology of these channels. In this study, we investigated the determinants of Nav1.8 expression in tsA201 cells, a transformed variant of HEK293 cells, using a combination of biochemistry, immunochemistry, and electrophysiology. Our data indicate that the unusually low expression levels of Nav1.8 in tsA201 cells results from a trafficking defect that traps the channel protein in the endoplasmic reticulum. Incubating the cultured cells with the local anesthetic lidocaine dramatically enhanced the cell surface expression of functional Nav1.8 channels. The biophysical properties of the heterologously expressed Nav1.8 channel are similar but not identical to those of the TTX-resistant Na current of native DRG neurons, recorded under similar conditions. Our data indicate that the lidocaine acts as a molecular chaperone that promotes efficient trafficking and increased cell surface expression of Nav1.8 channels.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

A high-capacity membrane potential FRET-based assay for NaV1.8 channels

Clinical treatment of neuropathic pain can be achieved with a number of different drugs, some of which interact with all members of the voltage-gated sodium channel (NaV1) family. However, block of central nervous system and cardiac NaV1 channels can cause dose-limiting side effects, preventing many patients from achieving adequate pain relief. Expression of the tetrodotoxin-resistant NaV1.8 subtype is restricted to small-diameter sensory neurons, and several lines of evidence indicate a role for NaV1.8 in pain processing. Given these features, NaV1.8 subtype-selective blockers are predicted to be efficacious in the treatment of neuropathic pain and to be associated with fewer adverse effects than currently available therapies. To facilitate the identification of NaV1.8-specific inhibitors, we stably expressed the human NaV1.8 channel together with the auxiliary human beta1 subunit (NaV beta1) in human embryonic kidney 293 cells. Heterologously expressed human NaV1.8/NaV beta1 channels display biophysical properties that are similar to those of tetrodotoxin-resistant channels present in mouse dorsal root ganglion neurons. A membrane potential, fluorescence resonance energy transfer-based functional assay on a fluorometric imaging plate reader (FLIPR-Tetra, Molecular Devices, Sunnyvale, CA) platform has been established. This highcapacity assay is sensitive to known state-dependent NaV1 modulators and can be used to identify novel and selective NaV1.8 inhibitors.

Characterization of voltage-gated sodium-channel blockers by electrical stimulation and fluorescence detection of membrane potential

Voltage-gated ion channels regulate many physiological functions and are targets for a number of drugs. Patch-clamp electrophysiology is the standard method for measuring channel activity because it fulfils the requirements for voltage control, repetitive stimulation and high temporal resolution, but it is laborious and costly. Here we report an electro-optical technology and automated instrument, called the electrical stimulation voltage ion probe reader (E-VIPR), that measures the activity of voltage-gated ion channels using extracellular electrical field stimulation and voltage-sensitive fluorescent probes. We demonstrate that E-VIPR can sensitively detect drug potency and mechanism of block on the neuronal human type III voltage-gated sodium channel expressed in human embryonic kidney cells. Results are compared with voltage-clamp and show that E-VIPR provides sensitive and information-rich compound blocking activity. Furthermore, we screened approximately 400 drugs and observed sodium channel-blocking activity for approximately 25% of them, including the antidepressants sertraline (Zoloft) and paroxetine (Paxil).

Analysis of human Nav1.8 expressed in SH-SY5Y neuroblastoma cells

The tetrodotoxin-resistant voltage-gated sodium channel alpha-subunit Nav1.8 is expressed in nociceptors and has been implicated in chronic pain. Difficulties of heterologous expression have so far precluded analysis of the pharmacological properties of human Nav1.8. To address this we have introduced human Nav1.8 in neuroblastoma SH-SY5Y cells. Voltage-clamp analysis showed that human Nav1.8 generated an inward tetrodotoxin-resistant sodium current with an activating threshold around -50 mV, half maximal activation at -11+/-3 mV and a reversal potential of 67+/-4 mV. These properties closely match those of the endogenous rat tetrodotoxin-resistant sodium current in dorsal root ganglia suggesting that the expressed human channel is in a near physiological conformation. Human Nav1.8 was resistant to tetrodotoxin and activated by the pyrethroid toxin deltamethrin. Both voltage-activated and deltamethrin-activated human Nav1.8 were inhibited by the sodium channel blockers BIII 890 CL, NW-1029, and mexiletine. Inhibition of Nav1.8 by these compounds may underlie their known analgesic effects in animal models.

Ambroxol, a Nav1.8-preferring Na(+) channel blocker, effectively suppresses pain symptoms in animal models of chronic, neuropathic and inflammatory pain

Neuropathic pain affects many patients, and treatment today is far from being perfect. Nav1.8 Na(+) channels, which are expressed by small fibre sensory neurons, are promising targets for novel analgesics. Na(+) channel blockers used today, however, show only limited selectivity for this channel subtype, and can cause dose-limiting side effects. Recently, the secretolytic ambroxol was found to preferentially inhibit Nav1.8 channels. We used this compound as a tool to investigate whether a Nav1.8-preferring blocker can suppress symptoms of chronic, neuropathic and inflammatory pain in animal models. The drug was tested in the formalin paw model, two models of mononeuropathy, and a model of monoarthritis in rats. Ambroxol's effects were compared with those of gabapentin. Ambroxol at a dose of 1g/kg had to be administered to rats to achieve the plasma levels that are reached in clinical use (for the treatment of infant and acute respiratory distress syndrome). Ambroxol (1g/kg) was only weakly effective in models for acute pain, but effectively reduced pain symptoms in all other models; in some cases it completely reversed pain behaviour. In most cases the effects were more pronounced than those of gabapentin (at 100mg/kg). These data show that a Nav1.8-preferring Na(+) channel blocker can effectively suppress pain symptoms in a variety of models for chronic, neuropathic and inflammatory pain at plasma levels, which can be achieved in the clinic.

Mexiletine treatment—induced inhibition of caspase-3 activation and improvement of behavioral recovery after spinal cord injury

Object. It has been demonstrated in several experimental studies that apoptosis contributes to cellular damage after spinal cord injury (SCI). During apoptosis dying cells secrete additional mediators of apoptosis such as cytokines and free radicals which have additional toxic effects and exacerbate neuronal death. The aim of this laboratory study was to investigate the effects of mexiletine on caspase-3 activation and functional recovery and compare its post-SCI effectiveness with methylprednisolone.

Methods. The rats were divided into five groups. Animals in the trauma group underwent traumatic interventions after laminectomy. Spinal cord contusion injury was produced using the weight-drop method. Animals in treatment groups received a single dose of methylprednisolone sodium succinate (Group C), single dose of mexiletine (Group D), or vehicle solution (saline; Group E) intraperitoneally immediately after injury. Hind-limb functions were assessed using the inclined plane technique and caspase-3 activity in tissue samples was measured 24 hours after SCI. Traumatic injury was found to increase tissue caspase-3 activity. In both treatment groups the drug prevented an increase in caspase-3 activity. Mexiletine treatment improved early behavioral recovery after SCI.

Conclusions. The results obtained in this study demonstrated that mexiletine treatment inhibits caspase-3 activation and preserve/restore better neuronal function compared with methylprednisolone after experimental SCI.