Na+ channel beta 1 subunit mRNA: differential expression in rat spinal sensory neurons

Functional modulation of the human voltage-gated sodium channel NaV1.8 by auxiliary β subunits

The voltage-gated sodium channel Na_v1.8 mediates the tetrodotoxin-resistant (TTX-R) Na⁺ current in nociceptive primary sensory neurons, which has an important role in the transmission of painful stimuli. Here, we describe the functional modulation of the human Na_v1.8 α-subunit in Xenopus oocytes by auxiliary β subunits. We found that the β3 subunit down-regulated the maximal Na⁺ current amplitude and decelerated recovery from inactivation of hNa_v1.8, whereas the β1 and β2 subunits had no such effects. The specific regulation of Na_v1.8 by the β3 subunit constitutes a potential novel regulatory mechanism of the TTX-R Na⁺ current in primary sensory neurons with potential implications in chronic pain states. In particular, neuropathic pain states are characterized by a down-regulation of Na_v1.8 accompanied by increased expression of the β3 subunit. Our results suggest that these two phenomena may be correlated, and that increased levels of the β3 subunit may directly contribute to the down-regulation of Na_v1.8. To determine which domain of the β3 subunit is responsible for the specific regulation of hNa_v1.8, we created chimeras of the β1 and β3 subunits and co-expressed them with the hNa_v1.8 α-subunit in Xenopus oocytes. The intracellular domain of the β3 subunit was shown to be responsible for the down-regulation of maximal Na_v1.8 current amplitudes. In contrast, the extracellular domain mediated the effect of the β3 subunit on hNa_v1.8 recovery kinetics.

Sodium channel β subunits: emerging targets in channelopathies

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in excitable cells. VGSCs in mammalian brain are heterotrimeric complexes of α and β subunits. Although β subunits were originally termed auxiliary, we now know that they are multifunctional signaling molecules that play roles in both excitable and nonexcitable cell types and with or without the pore-forming α subunit present. β subunits function in VGSC and potassium channel modulation, cell adhesion, and gene regulation, with particularly important roles in brain development. Mutations in the genes encoding β subunits are linked to a number of diseases, including epilepsy, sudden death syndromes like SUDEP and SIDS, and cardiac arrhythmia. Although VGSC β subunit-specific drugs have not yet been developed, this protein family is an emerging therapeutic target.

Differential expression of sodium channel β subunits in dorsal root ganglion sensory neurons

The small-diameter (<25 μm) and large-diameter (>30 μm) sensory neurons of the dorsal root ganglion (DRG) express distinct combinations of tetrodotoxin sensitive and tetrodotoxin-resistant Na(+) channels that underlie the unique electrical properties of these neurons. In vivo, these Na(+) channels are formed as complexes of pore-forming α and auxiliary β subunits. The goal of this study was to investigate the expression of β subunits in DRG sensory neurons. Quantitative single-cell RT-PCR revealed that β subunit mRNA is differentially expressed in small (β(2) and β(3)) and large (β(1) and β(2)) DRG neurons. This raises the possibility that β subunit availability and Na(+) channel composition and functional regulation may differ in these subpopulations of sensory neurons. To further explore these possibilities, we quantitatively compared the mRNA expression of the β subunit with that of Na(v)1.7, a TTX-sensitive Na(+) channel widely expressed in both small and large DRG neurons. Na(v)1.7 and β subunit mRNAs were significantly correlated in small (β(2) and β(3)) and large (β(1) and β(2)) DRG neurons, indicating that these subunits are coexpressed in the same populations. Co-immunoprecipitation and immunocytochemistry indicated that Na(v)1.7 formed stable complexes with the β(1)-β(3) subunits in vivo and that Na(v)1.7 and β(3) co-localized within the plasma membranes of small DRG neurons. Heterologous expression studies showed that β(3) induced a hyperpolarizing shift in Na(v)1.7 activation, whereas β(1) produced a depolarizing shift in inactivation and faster recovery. The data indicate that β(3) and β(1) subunits are preferentially expressed in small and large DRG neurons, respectively, and that these auxiliary subunits differentially regulate the gating properties of Na(v)1.7 channels.

Regulatory Role of Voltage-Gated Na Channel β Subunits in Sensory Neurons

Voltage-gated sodium Na(+) channels are membrane-bound proteins incorporating aqueous conduction pores that are highly selective for sodium Na(+) ions. The opening of these channels results in the rapid influx of Na(+) ions that depolarize the cell and drive the rapid upstroke of nerve and muscle action potentials. While the concept of a Na(+)-selective ion channel had been formulated in the 1940s, it was not until the 1980s that the biochemical properties of the 260-kDa and 36-kDa auxiliary β subunits (β(1), β(2)) were first described. Subsequent cloning and heterologous expression studies revealed that the α subunit forms the core of the channel and is responsible for both voltage-dependent gating and ionic selectivity. To date, 10 isoforms of the Na(+) channel α subunit have been identified that vary in their primary structures, tissue distribution, biophysical properties, and sensitivity to neurotoxins. Four β subunits (β(1)-β(4)) and two splice variants (β(1A), β(1B)) have been identified that modulate the subcellular distribution, cell surface expression, and functional properties of the α subunits. The purpose of this review is to provide a broad overview of β subunit expression and function in peripheral sensory neurons and examine their contributions to neuropathic pain.

Regulation of Nav1.6 and Nav1.8 peripheral nerve Na+ channels by auxiliary β-subunits

Voltage-gated Na(+) (Na(v)) channels are composed of a pore-forming α-subunit and one or more auxiliary β-subunits. The present study investigated the regulation by the β-subunit of two Na(+) channels (Na(v)1.6 and Na(v)1.8) expressed in dorsal root ganglion (DRG) neurons. Single cell RT-PCR was used to show that Na(v)1.8, Na(v)1.6, and β(1)-β(3) subunits were widely expressed in individually harvested small-diameter DRG neurons. Coexpression experiments were used to assess the regulation of Na(v)1.6 and Na(v)1.8 by β-subunits. The β(1)-subunit induced a 2.3-fold increase in Na(+) current density and hyperpolarizing shifts in the activation (-4 mV) and steady-state inactivation (-4.7 mV) of heterologously expressed Na(v)1.8 channels. The β(4)-subunit caused more pronounced shifts in activation (-16.7 mV) and inactivation (-9.3 mV) but did not alter the current density of cells expressing Na(v)1.8 channels. The β(3)-subunit did not alter Na(v)1.8 gating but significantly reduced the current density by 31%. This contrasted with Na(v)1.6, where the β-subunits were relatively weak regulators of channel function. One notable exception was the β(4)-subunit, which induced a hyperpolarizing shift in activation (-7.6 mV) but no change in the inactivation or current density of Na(v)1.6. The β-subunits differentially regulated the expression and gating of Na(v)1.8 and Na(v)1.6. To further investigate the underlying regulatory mechanism, β-subunit chimeras containing portions of the strongly regulating β(1)-subunit and the weakly regulating β(2)-subunit were generated. Chimeras retaining the COOH-terminal domain of the β(1)-subunit produced hyperpolarizing shifts in gating and increased the current density of Na(v)1.8, similar to that observed for wild-type β(1)-subunits. The intracellular COOH-terminal domain of the β(1)-subunit appeared to play an essential role in the regulation of Na(v)1.8 expression and gating.

Differential expression of neuronal voltage-gated sodium channel mRNAs during the development of the rat trigeminal ganglion

The developmental pattern of sodium channel expression in neurons of primary sensory ganglia is likely reflected in the electrical behavior of these cells. Little information is available on how voltage-gated sodium channels in sensory neurons are expressed during development in the trigeminal-innervated craniofacial region, where sensitivity is fundamental during early stages of life. Using in situ hybridization, we here demonstrate a differential both regional and transcript-dependent distribution of sodium channel alpha- and beta-subunits between Embryonic day (E)15 and Postnatal day (P)5/6 in the rat trigeminal ganglion. Na(v)1.3 mRNA was strongly expressed at E15, but declined to low levels at P5/P6. Na(v)1.8 was expressed at E15, increased to reach maximum levels at P1 and then decreased. Na(v)1.9 mRNA was detected at E19, reached a maximum at P1, and was then reduced. beta1 mRNA showed a steady rise in expression from E17, while beta2 mRNA was widely expressed from P1. beta 3 mRNA was detected at E15, reached a maximum at E19 followed by a decrease in expression. In the ophthalmic TG portion, there was a higher expression level of Na(v)1.8 and Na(v)1.9 between E19 and P5/P6 as compared to the maxillary/mandibular part, indicating an unexpected positional difference in channel distribution. mRNA levels of p11, which facilitates the expression of Na(v)1.8, were high at all stages. These findings show that trigeminal ganglion sodium channel transcripts mature in steps that are specific for each transcript. They also raise the possibility that different facial regions could differ in the ability to transmit sensory signals during early life.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Floxed allele for conditional inactivation of the voltage-gated sodium channel β1 subunitScn1b

The voltage-gated sodium channel gene Scn1b encodes the auxiliary subunit beta1, which is widely distributed in neurons and glia of the central and peripheral nervous systems, cardiac myocytes, skeletal muscle myocytes, and neuroendocrine cells. We showed previously that the Scn1b null mutation results in a complex and severe phenotype that includes retarded growth, seizures, ataxia, and death by postnatal day 21. We generated a floxed allele of Scn1b by inserting loxP sites surrounding the second coding exon. Ubiquitous deletion of the floxed exon by Cre recombinase using CMV-Cre-transgenic mice produced the Scn1b(del) allele. The null phenotype of Scn1b(del) homozygotes is indistinguishable from that of Scn1b nulls and confirms the invivo inactivation of Scn1b. Conditional inactivation ofthe floxed allele will make it possible to circumvent the lethality that results from complete loss of this gene, such that the physiological role of Scn1b in specific cell types and/or specific developmental time points can be investigated.

Expression of auxiliary β subunits of sodium channels in primary afferent neurons and the effect of nerve injury

Multiple voltage-gated sodium channels are the primary mediators of cell excitability. They are multimers that consist of the pore-forming alpha subunit and auxiliary beta subunits. Although ion permeability and voltage sensing are primarily determined by the alpha subunit, beta subunits are important modulators of sodium channel function. The purpose of this study was to assess the effect of axotomy on the expression of beta subunits (beta(1), beta(2) and beta(3)) and coexpression of Na(v)1.3 and beta(3) subunits in the dorsal root ganglion (DRG). We used sciatic nerve transection models or spared nerve injury (SNI) models in the rat. In reverse transcriptase-polymerase chain reaction analysis, there were no significant differences between contralateral and ipsilateral DRGs of beta(1) and beta(2) mRNA 3 days after axotomy. beta(3) mRNA expression in ipsilateral DRGs increased significantly compared with contralateral DRGs 3 days after axotomy. In in situ hybridization histochemistry, beta(1) mRNA was predominantly expressed in medium- to large-size neurons, whereas beta(2) mRNA was expressed in small- to large-size neurons. There were no significant differences in beta(1) and beta(2) mRNA between contralateral and ipsilateral DRGs 3 days after axotomy. In contrast, beta(3) mRNA was mainly expressed in small neurons and occasionally in medium- to large-size neurons, and beta(3) mRNA expression in small c-type neurons in ipsilateral DRGs was increased significantly compared with contralateral DRGs. We examined beta(3) mRNA expression with one of alpha subunits, Na(v)1.3-ir, in DRG neurons after axotomy using the double labeling method. We found a high percentage of coexpression in injured DRG neurons: 83.6+/-2.8% of neurons expressing beta(3) mRNA were labeled for Na(v)1.3-ir; 70.1+/-3.1% of Na(v)1.3-ir neurons expressed beta(3) mRNA. We also examined the expression of beta(3) mRNA in DRG neurons in the SNI model, a neuropathic pain model. We used activating transcription factor 3 to identify axotomized neurons, and found that beta(3) mRNA up-regulation occurred mainly in axotomized neurons in the neuropathic pain model. These data strongly suggest that beta(3) expression in injured DRG neurons following axotomy might be an important pathomechanism of post-nerve injury pain in primary sensory neurons.

Molecular diversity of structure and function of the voltage-gated Na+ channels

A variety of different isoforms of voltage-sensitive Na+ channels have now been identified. The recent three-dimensional analysis of Na+ channels has unveiled a unique and unexpected structure of the Na+ channel protein. Na+ channels can be classified into two categories on the basis of their amino acid sequence, Nav1 isoforms currently comprising nine highly homologous clones and Nax that possesses structure diverging from Nav1, especially in several critical functional motifs. Although the functional role of Nav1 isoforms is primarily to form an action potential upstroke in excitable cells, recent biophysical studies indicate that some of the Nav1 isoforms can also influence subthreshold electrical activity through persistent or resurgent Na+ currents. Nav1.8 and Nav1.9 contain an amino acid sequence common to tetrodotoxin resistant Na+ channels and are localized in peripheral nociceptors. Recent patch-clamp experiments on dorsal root ganglion neurons from Nav1.8-knock-out mice unveiled an additional tetrodotoxin-resistant Na+ current. The demonstration of its dependence on Nav1.9 provides evidence for a specialized role of Nav1.9, together with Nav1.8, in pain sensation. Although Nax has not been successfully expressed in an exogenous system, recent investigations using relevant native tissues combined with gene-targeting have disclosed their unique "concentration"-sensitive but not voltage-sensitive roles. In this context, these emerging views of novel functions mediated by different types of Na+ channels are reviewed, to give a perspective for future research on the expanding family of Na+ channel clones.

Gating properties of Na(v)1.7 and Na(v)1.8 peripheral nerve sodium channels

Several distinct components of voltage-gated sodium current have been recorded from native dorsal root ganglion (DRG) neurons that display differences in gating and pharmacology. This study compares the electrophysiological properties of two peripheral nerve sodium channels that are expressed selectively in DRG neurons (Na(v)1.7 and Na(v)1.8). Recombinant Na(v)1.7 and Na(v)1.8 sodium channels were coexpressed with the auxiliary beta(1) subunit in Xenopus oocytes. In this system coexpression of the beta(1) subunit with Na(v)1.7 and Na(v)1.8 channels results in more rapid inactivation, a shift in midpoints of steady-state activation and inactivation to more hyperpolarizing potentials, and an acceleration of recovery from inactivation. The coinjection of beta(1) subunit also significantly increases the expression of Na(v)1.8 by sixfold but has no effect on the expression of Na(v)1.7. In addition, a great percentage of Na(v)1.8+beta(1) channels is observed to enter rapidly into the slow inactivated states, in contrast to Nav1.7+beta(1) channels. Consequently, the rapid entry into slow inactivation is believed to cause a frequency-dependent reduction of Na(v)1.8+beta(1) channel amplitudes, seen during repetitive pulsing between 1 and 2 Hz. However, at higher frequencies (>20 Hz) Na(v)1.8+beta(1) channels reach a steady state to approximately 42% of total current. The presence of this steady-state sodium channel activity, coupled with the high activation threshold (V(0.5) = -3.3 mV) of Na(v)1.8+beta(1), could enable the nociceptive fibers to fire spontaneously after nerve injury.

beta3, a novel auxiliary subunit for the voltage-gated sodium channel, is expressed preferentially in sensory neurons and is upregulated in the chronic constriction injury model of neuropathic pain

Adult dorsal root ganglia (DRG) have been shown to express a wide range of voltage-gated sodium channel alpha-subunits. However, of the auxiliary subunits, beta1 is expressed preferentially in only large- and medium-diameter neurons of the DRG while beta2 is absent in all DRG cells. In view of this, we have compared the distribution of beta1 in rat DRG and spinal cord with a novel, recently cloned beta1-like subunit, beta3. In situ hybridization studies demonstrated high levels of beta3 mRNA in small-diameter c-fibres, while beta1 mRNA was virtually absent in these cell types but was expressed in 100% of large-diameter neurons. In the spinal cord, beta3 transcript was present specifically in layers I/II (substantia gelatinosa) and layer X, while beta1 mRNA was expressed in all laminae throughout the grey matter. Since the pattern of beta3 expression in DRG appears to correlate with the TTX-resistant voltage-gated sodium channel subunit PN3, we co-expressed the two subunits in Xenopus oocytes. In this system, beta3 caused a 5-mV hyperpolarizing shift in the threshold of activation of PN3, and a threefold increase in the peak current amplitude when compared with PN3 expressed alone. On the basis of these results, we examined the expression of beta-subunits in the chronic constriction injury model of neuropathic pain. Results revealed a significant increase in beta3 mRNA expression in small-diameter sensory neurons of the ipsilateral DRG. These results show that beta3 is the dominant auxiliary sodium channel subunit in small-diameter neurons of the rat DRG and that it is significantly upregulated in a model of neuropathic pain.

Schwann cells modulate sodium channel expression in spinal sensory neurons in vitro

In order to study the factors that govern the expression of sodium channel alpha-, beta1- and beta2-subunits, the influence that Schwann cells (SC) exert in the expression of sodium channels in DRG neurons was examined with in situ hybridization, immunocytochemistry, and patch clamp recording. The expression of sodium channel alpha-, beta1-, and beta2-subunit mRNAs in DRG neurons isolated from E15 rats cultured in defined medium in the absence (control) or presence of SC, or in SC-conditioned medium, was examined with isoform-specific riboprobes for sodium channel alpha-subunits I, II, III, NaG, Na6, hNE/PN1, SNS, and beta1- and beta2-subunits. DRG neurons cultured in the presence of SC displayed a significant (P < 0.05) increase in the hybridization signal for NaG, Na6, SNS, and Na beta2 mRNAs in comparison to control DRG neurons. In contrast, in SC-conditioned medium, only the hybridization signal for SNS mRNA was significantly increased. The upregulation of sodium channel mRNAs in DRG neurons co-cultured with SC was paralleled by an increase in sodium channel immunoreactivity of these cells. An increase in the mean sodium current density in DRG neurons in the presence of SC was also observed. These results demonstrate that a SC-derived factor selectively upregulates sodium channel alpha- and beta-subunit mRNAs in DRG neurons isolated from E15 rats that is reflected in an increase in functional sodium channels in these cells. This culture system may allow elucidation of the SC factor(s) that modulate the expression of sodium channels in DRG neurons.

Spinal sensory neurons express multiple sodium channel alpha-subunit mRNAs

The expression of sodium channel alpha-, beta 1- and beta 2-subunit mRNAs was examined in adult rat DRG neurons in dissociated culture at 1 day in vitro and within sections of intact ganglia by in situ hybridization and reverse transcription polymerase chain reaction (RT-PCR). The results demonstrate that sodium channel alpha-subunit mRNAs are differentially expressed in small (< 25 microns diam), medium (25-45 microns diam.) and large (> 45 microns diam.) cultured DRG neurons at 1 day in vitro (div). Sodium channel mRNA I is expressed at higher levels in large neurons than small DRG neurons, while sodium channel mRNA II is variably expressed, with most cells lacking or exhibiting low levels of detectable signal of these mRNAs and limited numbers of neurons with moderate expression levels. DRG neurons generally exhibit negligible or low levels of hybridization signal for sodium channel mRNA III. Sodium channel mRNAs Na6 and NaG show similar patterns of expression, with most large and many medium DRG neurons exhibiting high levels of expression. The mRNA for the rat cognate of human sodium channel hNE-Na is detected in virtually every DRG neuron; most cells in all size classes exhibit moderate or high levels of hNE-Na expression. Sodium channel SNS mRNA is expressed in all size classes of DRG neurons, but shows greater expression in small and medium DRG neurons than in large neurons. The mRNA for the rat cognate of mouse sodium channel mNa 2.3 is not detected, or is detected at low levels, in most DRG neurons, regardless of size, although moderate expression is detected in some neurons. Sodium channel beta 1- and beta 2-subunit mRNAs exhibit similar expression patterns; they are detected in most DRG neurons, although the level of expression tends to be greater in large neurons than in small neurons. RT-PCR and in situ hybridization of intact adult DRG showed a similar pattern of expression of sodium channel mRNAs to that observed in DRG neurons in vitro. These results demonstrate that adult DRG neurons express multiple sodium channel mRNAs in vitro and in situ and suggest a molecular basis for the biophysical heterogeneity of sodium currents observed in these cells.

Expression of mRNA for a sodium channel in subfamily 2 in spinal sensory neurons

RNA blot analysis and non-isotopic in situ hybridization cytochemistry were used to study the expression of the mRNA for the glial sodium channel NaG, belonging to Na+ channel subfamily 2, in rat dorsal root ganglia (DRG). mRNA hybridizing at high stringency with an antisense riboprobe against the NaG sequence was observed in both Schwann cells and spinal sensory neurons in situ within DRG, but was expressed at higher levels in the latter. In contrast, hybridization was not detectable in neurons within hippocampus, cerebellum and spinal cord. The expression of the mRNA hybridizing with the NaG probe appears to be developmentally regulated in both Schwann cells and DRG neurons, with levels increasing as development proceeds. Thus, in addition to the mRNAs for types I and II/IIA alpha-subunits and beta 1-subunit in DRG neurons and types II/IIA and III alpha-subunits beta 1-subunit in Schwann cells, the mRNA for an additional sodium channel belonging to subfamily 2 is expressed in these cells in situ.

Expression of sodium channel alpha- and beta-subunits in the nervous system of the myelin-deficient rat

Using subtype-specific riboprobes and a non-isotope in situ hybridization technique, the pattern of expression of the mRNAs for voltage dependent sodium channel alpha-subunits I, II, III and NaG, and the beta 1-subunit were compared in myelin-deficient rats and unaffected male littermates. Tissues examined included the hippocampus, cerebellum, spinal cord and dorsal root ganglia. Previous studies have demonstrated that the expression of sodium channel alpha- and beta 1-subunits follows a distinct temporal and spatial pattern during development, characterized in part by greater expression of alpha-subunit III and its mRNA during development than in the adult. We examined animals of 20-22 days of age, a time when, according to earlier reports, the unaffected animals should nearly have reached an adult expression pattern. Normal male littermates were indeed found to express a sodium channel subunit mRNA pattern generally consistent with previous reports on adult rats. Myelin-deficient animals exhibited an expression pattern identical to the unaffected littermates, indicating that myelination is not required for the progression from the embryonic to the adult expression pattern of sodium channel subunits.