ARTICLE, Development/Plasticity/Repair

Antisense Suppression of Potassium Channel Expression Demonstrates Its Role in Maturation of the Action Potential

Anne Vincent, Nathan J. Lautermilch and Nicholas C. Spitzer
Journal of Neuroscience 15 August 2000, 20 (16) 6087-6094; DOI: https://doi.org/10.1523/JNEUROSCI.20-16-06087.2000

Abstract

A developmental increase in delayed rectifier potassium current (IKv) in embryonicXenopus spinal neurons is critical for the maturation of excitability and action potential waveform. Identifying potassium channel genes that generate IKv is essential to understanding the mechanisms by which they are controlled. SeveralKv genes are upregulated during embryogenesis in parallel with increases in IKv and produce delayed rectifier current when heterologously expressed, indicating that they could encode channels underlying this current. We used antisense (AS) cRNA to test the contribution of xKv3.1to the maturation of IKv, becausexKv3.1 AS appears to suppress specifically heterologous expression of potassium current by xKv3.1 mRNA. The injection of xKv3.1 AS into embryos reduces endogenous levels of xKv3.1 mRNA in the developing spinal cord and reduces the amplitude and rate of activation ofIKv in 40% of cultured neurons, similar to the percentage of neurons in which endogenous xKv3.1transcripts are detected. The current in these mature neurons resembles that at an earlier stage of differentiation before the appearance ofxKv3.1 mRNA. Furthermore, AS expression increases the duration of the action potential in 40% of the neurons. No change in voltage-dependent calcium current is observed, suggesting that the decrease in IKv is sufficient to account for lengthening of the action potential. Computer-simulated action potentials incorporating observed reductions in amplitude and rate of activation of IKv exhibit an increase in duration similar to that observed experimentally. ThusxKv3.1 contributes to the maturation ofIKv in a substantial percentage of these developing spinal neurons.

Two general patterns of maturation of electrical excitability are apparent from investigation of embryonic nervous systems. In the first, long-duration principally Ca2+-dependent action potentials (APs) are converted to brief Na+-dependent impulses (Spitzer and Lamborghini, 1976; Gottmann et al., 1988) as a result of developmental increases in IK that are delayed in relation to the initial expression ofICa. In the second, APs are brief and Na+-dependent from the start (Bader et al., 1985) as a result of the early expression ofIK. The ratio of K+ to Ca2+current at early stages of differentiation determines whether or not APs have a prominent Ca2+ dependence (Lockery and Spitzer, 1992) that can be significant in regulating aspects of neuronal differentiation (Gu and Spitzer, 1995).

The requirement for transcription in the upregulation of delayed rectifier potassium current (IKv) that converts Ca2+- to Na+-dependent APs in Xenopusspinal neurons may include the expression of potassium channel genes, because Kv transcripts appear during the period of development of IKv. In vivo, transcripts encoding xKv1.2 are detected shortly after neural induction (Ribera, 1990), whereas mRNA encodingxKv1.1 appears at larval stages and is localized in Rohon–Beard primary sensory neurons in the amphibian spinal cord as well as in spinal and trigeminal ganglia (Ribera and Nguyen, 1993).xKv2.2 transcripts are expressed selectively in motoneurons (Burger and Ribera, 1996), and xKv3.1 mRNA appears first in interneurons and subsequently in other cells throughout the spinal cord (Gurantz et al., 2000). xKv1.1,xKv2.2, and xKv3.1 transcripts are upregulated inXenopus spinal neurons in vitro during the same embryonic period (Gurantz et al., 1996, 2000). The restricted localization and sustained presence of xKv3.1 mRNA in the spinal cord make it attractive for further study.

Antisense oligonucleotides have been used to suppress the appearance of delayed rectifier potassium channels in a rat pituitary cell line (Chung et al., 1995), indicating that antisense can be used to suppress the expression of voltage-gated channels as well as other components of differentiating neurons (Caceres and Kosik, 1990; Listerud et al., 1991; Wahlestedt et al., 1993; Yu et al., 1993). We used antisense (AS) cRNA to test the contribution of xKv3.1 to the maturation ofIKv. xKv3.1 AS transcripts suppress heterologous expression of potassium current in oocytes when coinjected with xKv3.1 mRNA but have no effect on currents generated when coinjected with xKv1.1, 2.1, or4.3 mRNA. Injection of xKv3.1 AS mRNA into one cell of a two-cell stage embryo followed by in situhybridization demonstrates that the endogenous level ofxKv3.1 mRNA is reduced. AS injection reduces the amplitude and the rate of activation of IKv in 40% of cultured neurons, similar to the percentage of neurons expressing xKv3.1 transcripts (Gurantz et al., 2000). AS expression also increases the duration of the action potential in 40% of neurons. The absence of an effect on calcium current and the increase in duration of APs in computer simulations modeling the effects of xKv3.1 AS on IKvindicate that xKv3.1 contributes to the maturation of the AP in these differentiating spinal neurons.

MATERIALS AND METHODS

Oocyte injection and electrophysiological recording.Adult female Xenopus laevis were anesthetized by immersion in a 0.03% solution of benzocaine (Sigma, St. Louis, MO) for 10 min, and ovarian lobes were removed surgically. Stage V–VI oocytes were defolliculated by gentle rocking in the presence of collagenase B (2 mg/ml; Roche Pharmaceuticals, Basel, Switzerland) in OR-2 [containing (in mm) 82.5 NaCl, 2 KCl, 1 MgCl2, and 5 HEPES, pH 7.5] for 60–90 min. Oocytes were stored overnight in ND-96 [containing (in mm) 96 NaCl, 2 KCl, 1.8 CaCl2, 1 MgCl2, and 5 HEPES, pH 7.5, supplemented with 550 mg/l sodium pyruvate, 0.5 mm theophylline, and 50 μg/ml gentamycin] and injected 18–24 hr after defolliculation, using micropipettes (10 μm tip diameter; Sutter Instruments, Novato, CA). Capped mRNA was generated from DNA constructs using the mMessage mMachine (Ambion, Austin, TX). xKv1.1 and 2.1cDNAs were kindly provided by Dr. A. B. Ribera (University of Colorado), and full-length cRNAs of xKv1.1, 2.1,3.1, and 4.3 were synthesized (Ribera and Nguyen, 1993; Burger and Ribera, 1996; Lautermilch and Spitzer, 1997; Gurantz et al., 2000). xKv3.1 AS was generated from a 3′ 1812 bp partial clone (Gurantz et al., 2000) by in vitrotranscription. mRNA (10–50 ng) was injected per oocyte, and currents were recorded 2–7 d after injection using standard two-electrode voltage-clamp techniques. Data were acquired with an Axoclamp 2A amplifier and an Axolab 1100 A/D converter (Axon Instruments, Burlingame, CA), using pClamp 5. Voltage and current electrodes were filled with 3 m KCl and had resistances of 0.5–1 MΩ. All recordings were made at 20–22°C, sampled at 300 μsec intervals, and filtered at 1 kHz with a Frequency Devices filter. Oocytes were perfused constantly during recordings; bath changes were achieved with >10 vol of new extracellular solution. The standard extracellular recording solution contained (in mm) 80 NaCl, 3 KCl, 5 MgCl2, and 5 HEPES, pH 7.4. TTX (10-6m; Sigma) and 10 mm CoCl2 were added to block voltage-gated Na+ and Ca2+ currents, respectively. Outward potassium currents were elicited by stepping to voltages from −40 to +80 or +90 mV from a holding potential of −80 mV to activate the conductances fully (Ribera and Nguyen, 1993; Burger and Ribera, 1996;Lautermilch and Spitzer, 1997; Gurantz et al., 2000). Data were analyzed with pClamp 5; currents were normalized and plotted as mean ± SEM. Percentage changes were calculated at each potential and averaged. Student's t test was used to determine statistical significance.

Embryo injection. RNA was generated as above and injected into developing embryos at one side of the animal pole immediately after first cleavage. Five to 10 nl of solution containing lineage tracer (20–30 mg/ml of FITC-conjugated dextran; Molecular Probes, Eugene, OR) and the desired RNA (5–200 pg/nl) were pressure-injected (2–3 psi for 2–3 sec). Coinjected dextran was pretreated to remove RNase contamination by boiling for 10 min in 0.1% pyrocarbonate, followed by methanol precipitation. Injection of one cell of the two-cell stage embryo permitted one-half of the embryo to serve as an uninjected internal control. On the basis of external morphological criteria the injected embryos underwent normal gastrulation, and survival was not impaired.

Whole-mount in situ hybridization. The nonradioactive detection method using digoxygenin-labeled probes (Harland, 1991; Ferreiro et al., 1992) was used with modifications (Burger and Ribera, 1996). xKv3.1 RNA probes were synthesized by transcription of the partial clone (Gurantz et al., 2000) in the presence of digoxygenin-labeled UTP (Roche Pharmaceuticals). Sense and antisense probes lacked 280 bp of the 5′ open reading frame (ORF) of the full-length xKv3.1. Injected and control albino embryos were fixed at embryonic stages of interest in MEMPFA (3.7% paraformaldehyde, 0.1 m MOPS, pH 7.4, 1 mm MgSO4, and 2 mm EGTA) at 4°C overnight. Hybridization was performed for 16 hr at 60°C. After incubation with antibody, washes were carried out over a 24 hr period with a minimum of five changes; the final wash was carried out overnight at 4°C. The alkaline phosphatase (AP) reaction product was developed in the presence of purple precipitating AP substrate (Roche Pharmaceuticals) for at least 12 hr. Whole-mount embryos were cleared in Murray's solution (2:1 benzyl benzoate/benzyl alcohol) and photographed with Kodak Ektachrome 160T slide film.

Culture preparation and electrophysiological recording.Embryos were produced by standard in vitrofertilization and staged according to Nieuw-koop and Faber (1967). Neural plate (stage 15) cultures were prepared (Spitzer and Lamborghini, 1976; Blair, 1983; Ribera and Spitzer, 1989) from the posterior two-thirds of the neural plate, which comprises the future spinal cord and hindbrain (Eagleson and Harris, 1990). Cells from a single embryo were plated on 35 mm tissue culture dishes and maintained in defined medium [containing (in mm) 116 NaCl, 0.67 KCl, 1.31 MgSO4, 10 CaCl2, and 4.6 Tris buffer, pH 7.8]. The cultures contained a mixed population of sensory, motor, and interneurons (Spitzer and Lamborghini, 1976; Bixby and Spitzer, 1984) as well as morphologically undifferentiated cells. Because the somitic mesoderm was not removed from the neural plate, the cultures contained myocytes as well. Whole-cell gigohm seal recording methods were used (Hamill et al., 1981). Neurons were identified on the basis of morphology, and those with short processes (<50 μm) were selected (O'Dowd et al., 1988) to avoid inadequate control of the membrane voltage. An Axopatch 1-D amplifier was used in conjunction with a TL-1 data interface and pClamp 5 (Axon Instruments) for acquisition of data. Electrodes were pulled (Sutter Instruments, Novato, CA) from borosilicate glass (Drummond Scientific, Broomall, PA) and had resistances ranging between 2 and 4 mΩ when filled with the standard pipette solution [containing (in mm) 100 KCl, 10 EGTA, and mm HEPES, pH 7.4]. Neurons were examined at 21°C using Nomarski optics at 400× magnification. Visualization of fluorescence did not interfere with cell survival or subsequent differentiation.

Whole-cell voltage-clamped potassium currents were recorded in saline consisting of (in mm) 80 NaCl, 3 KCl, 5 MgCl2, 10 CoCl2, and 5 HEPES, pH 7.4. Sodium currents were blocked by the addition of 10-6m TTX; calcium currents were avoided by the omission of CaCl2 from the saline and the addition of 10 mmCoCl2. Cells were held at −80 mV and stepped to potentials between −40 and +60 mV for 600 msec. In other experiments the calcium current was recorded in the presence of TTX and 10 mm CaCl2; MgCl2and CoCl2 were omitted from the bath. Pipettes were filled with (in mm) 100 CsCl, 10 tetraethylammonium-Cl (TEA), 10 EGTA, and 10 HEPES, pH 7.4. Membrane potential usually was held at −80 mV and stepped to depolarized voltages ranging between −70 and +50 mV in increments of 10 mV for 50 msec. Action potentials were elicited with brief 2.5 msec depolarizing current pulses; a steady level of current was injected to hold the resting membrane potential near −60 mV. For these experiments the cells were bathed in a saline solution consisting of (in mm) 125 NaCl, 3 KCl, 10 CaCl2, and 5 HEPES, pH 7.4.

The capacitive transient was nulled electronically before establishment of the whole-cell configuration, and currents and potentials were digitized at 100 and 50 μsec. Cell capacitance and series resistance were determined from the capacitive current transient recorded after break in, sampled at 20 μsec intervals. Cell capacitance was used to evaluate the membrane surface area (1 μF/cm2) for normalization of current amplitude to density. Leak subtraction was accomplished by using aP/−x protocol, with x = 11 hyperpolarizing subpulses (pClamp 5, Axon Instruments). Evaluation of series and input resistance and cell capacitance was aided by a program written and generously provided by Dr. Devorah Gurantz (University of California, San Diego). Measurement of times to peak or half-maximum current was accomplished with Clampfit 6. Data are presented as mean ± SEM. The statistical significance of differences between two groups of data was assessed using the nonparametric Mann–WhitneyU test, and the significance of multiple comparisons was evaluated with an ANOVA (Bonferroni's post hoc analysis);p values < 0.05 were considered statistically significant.

Population analysis. Approximately 40% of neurons expressxKv3.1 transcripts (Gurantz et al., 2000); thusIKv was not expected to be affected in all cells that are fluorescent and contain xKv3.1 AS. In voltage-clamp experiments the fluorescent neurons were assigned to affected (Fas*) and unaffected categories (Fas) on the basis of two criteria: a decrease in density of IKv and a decrease in its activation kinetics (increase in time to half-maximal current). When values for both at potentials of +20, +40, and +60 mV exceeded 2 SD from mean values for nonfluorescent (NF) control neurons recorded in the same culture dish, they were designated FAS*; for a normal distribution only 5% of cells are expected to be in this category. The remaining neurons were designated FAS. The percentage of change inIKv density and activation kinetics was calculated at each potential and averaged. For experiments in which action potentials (AP) were recorded, assignment was determined by the measurement of AP duration at 0 mV, following the same convention, and the mean percentage of change was calculated.

RESULTS

Injection of xKv3.1 AS RNA specifically suppressesxKv3.1 current in oocytes

On the basis of previous work demonstrating xKv3.1transcripts in ∼40% of cultured neurons (Gurantz et al., 2000), we sought to identify their contribution to the maturation ofIKv by using AS RNA. The specificity of xKv3.1 AS was examined in oocytes by coinjection of RNA encoding a member of each of the four subfamilies of voltage-activated potassium channels presently cloned from Xenopus. Currents elicited from oocytes injected with xKv3.1 mRNA were reduced by 86% when fourfold more xKv3.1 AS RNA was coinjected (Fig. 1A;p < 0.01), with no apparent effect on the current–voltage relationship. In contrast, coinjection of the same stoichiometric ratio of xKv3.1 AS RNA with cRNA encodingxKv1.1, 2.2, and 4.3 did not decrease peak current or affect their current–voltage relationships (Fig.1B–D). These data suggest that xKv3.1 AS RNA specifically reduces xKv3.1 current in oocytes.

Fig. 1.
Fig. 1.

Antisense (AS) RNA to xKv3.1specifically inhibits the current expressed in oocytes injected withxKv3.1 mRNA. Currents were recorded after injection of cRNA encoding xKv genes ± xKv3.1AS. Currents recorded in the presence of AS were normalized to those recorded from oocytes of the same clutch in the absence of AS.A, xKv3.1 AS was injected at a stoichiometry of 4:1 with xKv3.1 RNA. AS decreased the current by 86% without shifting the I–V relation.B–D, xKv3.1 AS had no significant effect on currents encoded by xKv1.1, xKv2.1, orxKv4.3 when mRNA was injected at the same stoichiometry.Circles, xKv RNA alone;squares, xKv RNA plusxKv3.1 AS. n ≥5 for each gene.

xKv3.1 AS reduces expression of endogenousxKv3.1 mRNA

In vivo expression of xKv3.1 was observed by whole-mount in situ hybridization with an AS probe for the first time at embryonic stage 22 in the anterior part of the spinal cord, consistent with previous observations (Gurantz et al., 2000). No reaction product was observed of embryos probed with xKv3.1sense RNA (Fig. 2A). Expression of the gene increased into the posterior spinal cord with further development and spanned its entire length by stage 30 (Fig.2B). After injection of one cell of a two-cell stage embryo with xKv3.1 AS RNA, these transcripts were visualized in the spinal cord using a sense probe (Fig. 2C). Moreover, transcripts were detected only on one side of the spinal cord, consistent with each cell of a first cleavage stage embryo, giving rise to the majority of the right or left side of the embryo (Klein, 1987). Restriction of AS to dorsal structures may be attributable to the exclusion of message from the yolk-rich gut region or to specific degradation. The detection of exogenous xKv3.1 AS RNA in a stage 30 (∼1.5 d) embryo is concordant with the long half-life of injected RNA (6–8 hr; Harland and Misher, 1988; Kintner, 1988). AS-injected embryos hybridized with an antisense probe revealed that endogenous xKv3.1 expression was reduced in one-half of the embryo, coincident with the presence of the injected AS RNA (Fig.2D); expression was not reduced on the side derived from the uninjected blastomere. These results demonstrate (1) that injected xKv3.1 AS RNA is present throughout the developmental period examined, concurrent with upregulation ofxKv3.1, (2) that the cRNA injection protocol deliversxKv3.1 AS to the spinal cord, and (3) that thexKv3.1 AS leads to a decrease in xKv3.1 mRNA on the injected side of the embryo.

Fig. 2.
Fig. 2.

Inhibition of expression of xKv3.1transcripts by antisense RNA injection is visualized by in situ hybridization of whole-mount Xenopusembryos with a xKv3.1-specific antisense probe. A, Sense (S) probe gives no signal in uninjected control embryos. B, AS probe reveals that endogenousxKv3.1 mRNA is present in the spinal cord of control embryos at stage 22 and thereafter (Gurantz et al., 2000).C, S probe labeling of AS-injected embryos demonstrates stability of injected xKv3.1 AS. Signal is observed in 16-cell through stage 30 embryos; dorsal view of another embryo indicates that only the injected side of the embryo (top) contains AS. D, Injection ofxKv3.1 AS reduces the endogenous signal detected with the AS probe. Dorsal view of another embryo indicates that reduction of the in situ hybridization signal is restricted to the injected side (top, arrowheads). Area inbox is enlarged at right. For all embryos anterior is to the right and dorsal isup. Scale bar, 1 mm. Blackspots on tailbud embryos are melanocytes.

Whole-cell recordings of IKv demonstrate that AS xKv3.1 RNA reduces peak current and activation kinetics in a subpopulation of neurons

IKv develops in cultured neurons (Barish, 1986; O'Dowd et al., 1988) in a manner similar to its development in vivo (Desarmenien et al., 1993). Differentiation of IKv requires the synthesis of new mRNA during a critical period (Ribera and Spitzer, 1989). xKv3.1 transcripts appear early during development, encode delayed rectifier potassium current, and are found in ∼40% of primary spinal neurons by 18–22 hr in culture (Gurantz et al., 2000). Does expression of the xKv3.1 gene play a critical role in the upregulation of IKv? To address this issue, we coinjected one cell of two-cell stage embryos withxKv3.1 AS RNA with a fluorescent FITC-dextran tracer and prepared cultures from neural plate stage embryos. We then examined whole-cell currents in individual neurons at 1 d in vitro when IKv normally has achieved its mature density and kinetics of activation (O'Dowd et al., 1988).

Injection of xKv3.1 AS RNA reducesIKv in a subpopulation of neurons, consistent with the presence of AS in neurons and suppression of the levels of xKv3.1 RNA (Fig. 2). The tracer is found in all cell types, including neurons, myocytes, and morphologically undifferentiated cells. Currents were elicited from neurons held at −80 mV and stepped to voltages from −40 to +60 mV. Outward current was detected first at steps to 0 mV; steady-state currents were measured and plotted against command potentials. Whole-cell recordings from nonfluorescent (NF) neurons confirmed previous results obtained from uninjected embryos (Gurantz et al., 2000), validating their use as internal controls. Recordings ofIKv from fluorescent (F) neurons revealed two subpopulations when compared with NF controls (Fig.3). Preliminary analysis indicated that both the density and rate of activation ofIKv were reduced in 40% of F cells. Accordingly, we sorted F cells into two categories: affected (FAS*, 40%) and unaffected (FAS, 60%), on the basis of differences in density and time to half-maximal activation (t1/2max) >2 SD from values for NF cells recorded from the same culture dish (Fig.4). Because neurons were analyzed in different cultures on different days, comparisons of density andt1/2max were made for each day to partition the neurons into FAS and FAS* categories. Thus some of the data for the NF and FAS* categories overlap in the aggregate data set illustrated in these figures. Significantly, although the amount of AS inhibition varied within the FAS*subpopulation, the percentage of cells in the affected category is similar to the percentage of neurons that have been shown to expressxKv3.1 transcripts (Gurantz et al., 2000). A 62 ± 1% reduction in density of IKv is seen in FAS* neurons (Fig. 4A, Table1). A correlated decrease int1/2max ofIKv is observed in FAS* neurons, andt1/2max is 166 ± 6% of control (Fig. 4B, Table 1). In FASneurons IKv density andt1/2max are not significantly different from that recorded from NF control neurons. These results suggest that xKv3.1 normally contributes to the upregulation of IKv in a subpopulation of spinal neurons.

Fig. 3.
Fig. 3.

Inhibition of expression ofIKv in cultured neurons after injection ofxKv3.1 AS RNA into a single blastomere of two-cell stage embryos. FITC-dextran was coinjected to identify neurons containing AS. Whole-cell currents elicited from a holding potential of −80 mV and steps from −40 to +60 mV reveal two subpopulations of neurons. One population, FAS* (fluorescent AS-injected, affected) neurons, has reducedIKv amplitude and activation kinetics. The other, FAS (fluorescent AS-injected, unaffected) neurons, exhibits currents that are not different from those of nonfluorescent (NF) controls. The records are from three neurons at 18–22 hr in culture.

Fig. 4.
Fig. 4.

Properties of IKv in FAS* neurons. Currents were recorded at 18–22 hr in culture. A, Current densities of FAS*neurons are reduced relative to those of NF and FASneurons. FAS* cells show a mean 62 ± 1% decrease in current density at 0 to +60 mV (p < 0.01), whereas FAS cells have currents that are not different from NF controls. The boxes include all measurements from cells at a particular voltage. Horizontal bars indicate the means for these measurements, and vertical barsindicate SEMs. NF, Nonfluorescent;FAS, fluorescent, but unaffected by AS;FAS*, fluorescent, affected by AS. Values were designated FAS* when they exceeded 2 SD from the mean values for NF neurons in the same culture dishand thus in some cases overlap the values for NF neurons recorded in other cultures; n = 27. B, Times to half-activation (t1/2max) of FAS* neurons are increased relative to those of NF and FAS neurons. FAS* cells show a 66 ± 6% increase in t1/2max at +20 to +60 mV (p < 0.01), whereas FAS cells have currents that are not different from NF controls. Values at 0 mV are not shown because IKv is small and slowly activating at this potential in FAS* neurons, and discrimination from noise is ambiguous; n = 27. Noise precluded measurement from two cells at +20 mV.

View this table:
Table 1.

Properties of control and antisense-treated neurons

Uncapped sense xKv3.1 mRNA does not affect expression of IKv

To test further the specificity of xKv3.1 AS in decreasing IKv, we injected sense RNA with sequence complementary to that of the AS. This RNA was not capped and did not contain a translational start site, to prevent expression of exogenous xKv3.1. Injection of one cell of a two-cell stage embryo with xKv3.1 sense RNA and a fluorescent tracer allowed visualization of cells that received the sense transcripts. The density of IKv in FS cells is not different from that in NF controls (Fig. 5A;p > 0.7, NS). Additionally, measurements of activation kinetics of IKv demonstrate that FS and NF control cells are not different (Fig.5B; p > 0.5, NS). These data indicate thatxKv3.1 sense RNA has no effect onIKv density and activation kinetics and argue that the effects of xKv3.1 AS are not attributable to spurious toxicity from RNA injection.

Fig. 5.
Fig. 5.

Injection of xKv3.1 sense RNA into a single blastomere of two-cell stage embryos has no effect on the expression of IKv in cultured neurons.A, Current densities of FS and NF neurons are not different. B, Times to half-activation of FS and NF neurons are not different. Conventions are as in Figure 4; n = 18.

AS suppression of xKv3.1 RNA results in longer action potentials

Because the injection of xKv3.1 AS RNA demonstrates that xKv3.1 transcripts contribute significantly toIKv in a subset of neurons, we determined whether its effect extended to the action potential (AP) as well. We predicted that the AP would increase in duration (Lockery and Spitzer, 1992) in the same percentage of neurons that undergo a decrease in IKv after injection ofxKv3.1 AS RNA. Using current-clamp recording, we depolarized neurons to elicit APs (Fig.6). Analysis of F neurons revealed that 40% had an AP duration >2 SD longer than that of NF controls (Fig.7), equal to the percentage of neurons that exhibited a decrease in amplitude and increase int1/2max ofIKv as a result of injection ofxKv3.1 AS RNA and similar to the percentage of neurons normally expressing xKv3.1 transcripts (Gurantz et al., 2000). The average duration of APs in NF neurons was 1.9 ± 0.2 msec, whereas that in FAS* neurons was 3.5 ± 0.6 msec (Table 1), resulting in a mean increase of 184% and demonstrating a substantial contribution of xKv3.1 to neuronal repolarization during an AP in these cells. In contrast, the duration of APs in FAS neurons was 1.9 ± 0.2 msec, not significantly different from that of NF controls (p > 0.05).

Fig. 6.
Fig. 6.

xKv3.1 AS increases action potential duration. The AP of FAS* neurons is longer in duration than that of NF and FAS neurons (p < 0.01), consistent with reduction of the amplitude of IKv. The records are from three neurons at 1 d in culture; n = 24.

Fig. 7.
Fig. 7.

The percentage of neurons in whichIKv and APs are affected byxKv3.1 AS is similar to the percentage of neurons previously shown to express xKv3.1 transcripts (Gurantz et al., 2000). Numbers in parenthesesindicate the number of neurons affected/tested for each condition.

xKv3.1 AS RNA does not reduceICa

The increase in AP duration after injection of xKv3.1AS RNA could result from either a decrease in outward current or an increase in sustained inward current. Accordingly, we examined calcium current (ICa) in F and NF neurons at 1 d in culture. Both low voltage- and high voltage-activatedICa were elicited by depolarization to potentials from −70 to +50 mV from a holding potential of −100 mV (Fig. 8A) (Gu and Spitzer, 1993). Comparison of currents in F and NF neurons reveals thatICa is not changed because of the presence of xKv3.1 AS RNA (Fig. 8B), suggesting that the increased duration of the AP is attributable to the effect on the expression of xKv3.1 alone (Table 1). Furthermore, these data reinforce the conclusion that thexKv3.1 gene product contributes significantly to the AP as well as to IKv.

Fig. 8.
Fig. 8.

ICa is not affected by the expression of xKv3.1 AS. A, Whole-cell voltage-clamp records of low voltage-activated and high voltage-activated calcium currents; potassium and sodium currents were suppressed pharmacologically. The records are from two neurons at 1 d in culture. Cells were held at −80 mV and stepped to potentials from −50 to +20 mV. B, Current densities for NF and FAS neurons. The records are from six and seven neurons at 1 d in culture. Conventions are as in Figure4. Mean ICa is not significantly different for these two populations. The apparent reduction in the density of calcium current at −10 mV likely reflects outliers in the NF population.

Action potentials simulated with reducedIKv are similar to those recorded in the presence of xKv3.1 AS RNA

Specific suppression of the amplitude andt1/2max activation ofIKv in a subset of neurons byxKv3.1 AS RNA is accompanied by an increase in the duration of the AP. To determine whether the reduction inIKv is sufficient to account for the increase in impulse duration, we performed computer simulations of the AP, using a program previously developed to examine the contribution of different currents to the generation of the AP (Lockery and Spitzer, 1992; Gu and Spitzer, 1993). Simulations of voltage-clampedIKv andICa mimic recorded currents (Fig.9A,B). Simulation of the AP in control neurons, using values for voltage-clamped currents recorded at 1 d in culture, yields an impulse with a duration of 2.0 msec at 0 mV, consistent with previous modeling results (Fig. 9C) (Lockery and Spitzer, 1992). When the amplitude ofIKv is reduced by 62% and the time to half-maximal is increased consistent with experimental recordings, the duration of the AP is 3.5 msec (Fig. 9C; 175% increase). Thus the extent of suppression of IKvwith xKv3.1 AS RNA is appropriate to account for the increase in AP duration recorded experimentally.

Fig. 9.
Fig. 9.

Computer-simulated action potentials in whichIKv is reduced by the amounts observed experimentally are longer in duration, consistent with the increase in experimentally recorded AP duration after the suppression ofIKv with xKv3.1 AS.A, Computer simulations ofIKv in control and experimental conditions (100% IKv, and with reduction in density and increase in time to half-maximal activation values derived from AS experiments). B, Simulations ofICa in control and experimental conditions; the jagged nature of the traces is attributable to the plotting regime, which increases the distance between points when the rate of change of the current is small. C, Simulations of APs in control and experimental conditions. Modeling was performed by using a previously developed program (Lockery and Spitzer; 1992; Gu and Spitzer, 1993). Mimicking experimental recording conditions, the holding potential was set at −60 mV, and brief 2.5 msec depolarizing current pulses were used to elicit APs arising after the termination of the stimulus.

DISCUSSION

Specificity of AS suppression of xKv3.1

Contribution of a channel gene to current expression requires that synthesis of transcripts precedes appearance of the current (Ribera, 1990; Ribera and Nguyen, 1993; Burger and Ribera, 1996; Gurantz et al., 1996; Baranauskas et al., 1999) and that suppression of the current occurs when transcripts are removed or functional channels are eliminated specifically (Ribera, 1996). Previous work showed thatxKv3.1 mRNA is present during the period in whichIKv matures (Gurantz et al., 2000). Here we show that expression of xKv3.1 AS RNA suppressesIKv, and conclude that transcription of the xKv3.1 potassium channel gene contributes to the maturation of IKv in a subset of embryonic spinal neurons. This conclusion relies on the specificity ofxKv3.1 AS for xKv3.1 mRNA, which is supported by three lines of evidence. First, xKv3.1 AS suppresses current encoded by xKv3.1 transcripts expressed in oocytes and has no effect on currents expressed by members of three other Kvsubfamilies. Second, xKv3.1 AS both reducesIKv and increases AP duration in the same percentage of neurons in which endogenous xKv3.1transcripts are expressed, making it unlikely that other genes of theKv3 subfamily are affected. Third, uncappedxKv3.1 sense RNA has no effect on the expression ofIKv. In situ hybridization of cultured neurons or staining with antibodies could provide structural corroboration of the functional result of the application of AS. This demonstration of the role of a specific member of theKv3 subfamily in the maturation ofIKv in 40% of Xenopusspinal neurons is complementary to the demonstration of the role ofKv1 subfamily members, using a dominant-negative strategy (Ribera, 1996).

Contribution of potassium channel genes in mature nervous systems

Functional properties of specific endogenously expressed potassium channels were identified first in a clonal rat pituitary cell line using AS oligonucleotides (Chung et al., 1995). Subsequently, the role of specific delayed and inward rectifier potassium channel genes in cardiac muscle was defined by suppression of activity of specific genes with AS (Feng et al., 1997; Nakamura et al., 1998; Bou-Abboud and Nerbonne, 1999; Wang et al., 1999). Suppression of A-type potassium current with AS has been reported to suppress LTP without affecting rat spatial maze learning or memory (Meiri et al., 1998). The function ofKv and G-protein-gated Kir genes in neuronal spike waveform, excitability, synaptic transmission, and nociception in postnatal mice has been determined by the deletion of these genes (Signorini et al., 1997; Clark and Tempel, 1998; Giese et al., 1998;Smart et al., 1998; Zhou et al., 1998, 1999). Here we demonstrate the role of transcripts of the xKv3.1 gene encoding potassium channels in the embryonic development of excitability by suppression of their activity with AS. The Kv3.1 gene also is expressed in developing inferior colliculus, auditory rhombencephalon, and cerebellum (Liu and Kaczmarek, 1998; Hendriks et al., 1999; Shibata et al., 1999) and seems likely to regulate excitability in these systems as well. Because spinal neurons exhibit spontaneous calcium transients that depend on transiently expressed calcium-dependent action potentials (Ca2+ spikes; Gu et al., 1994), the suppression of Kv3.1 andIKv could extend the developmental period during which these transients are expressed and thereby affect neuronal differentiation (Gu and Spitzer, 1995).

Heterogeneity of potassium channel gene expression

Heterogeneity of potassium channel gene expression inXenopus spinal neurons is suggested by examination of transcripts and by the results of eliminating their contribution toIKv. In situ hybridization and single-cell RT-PCR demonstrate that different Kv genes are expressed in subsets of neurons (Ribera and Nguyen, 1993; Burger and Ribera, 1996; Gurantz et al., 1996, 2000). Using a dominant-negative construct to suppress the activity of thexKv1 subfamily (Ribera, 1996), the current was reduced to varying degrees in 60% of neurons expressing the construct, implying coexpression of Kv genes of other subfamilies. In addition, 20% of neurons expressing the construct had no detectableIKv, consistent with expression of thexKv1 subfamily alone in these neurons. These conclusions are supported by the finding that channel subunits of one subfamily do not coassemble with those from other subfamilies (Covarrubias et al., 1991;Xu et al., 1995) (however, see Shahidullah et al., 1995; Chen et al., 1996; Kerschensteiner and Stocker, 1999). Using antisense to suppress the activity of xKv3.1, we show thatIKv is reduced to 38% of its amplitude in 40% of neurons, consistent with the expression of transcripts encoded by this gene (Gurantz et al., 2000) and suggesting that the antisense is both specific and suppresses the contributions from all xKv3.1 transcripts. On the other hand, no neurons that were assayed completely lackedIKv. These observations raise the question of the difference in functional roles of these multiple potassium channels, which generate similar currents in heterologous systems. Future studies will examine differences in modulation and localization of these otherwise apparently redundantly expressed channels.

Footnotes

  • This work was supported by National Science Foundation Grant IBN-9603978 and National Institutes of Health, National Institute of Neurological Diseases and Stroke Grant NS07220. We thank the members of our laboratory for discussions; Steve Watt for technical support; I-Teh Hsieh for assistance with computer simulations; Anna Marnick for library research; and Tim Gomez, Devorah Gurantz, Paul Kingston, and Ray Smith for comments on this manuscript.

    Correspondence should be addressed to Dr. Nicholas C. Spitzer, Department of Biology 0357, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0357. E-mail: nspitzer{at}ucsd.edu.

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Antisense Suppression of Potassium Channel Expression Demonstrates Its Role in Maturation of the Action Potential
Anne Vincent, Nathan J. Lautermilch, Nicholas C. Spitzer
Journal of Neuroscience 15 August 2000, 20 (16) 6087-6094; DOI: 10.1523/JNEUROSCI.20-16-06087.2000
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