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Genomic Organization, Chromosomal Localization, Tissue Distribution, and Biophysical Characterization of a Novel MammalianShaker-related Voltage-gated Potassium Channel, Kv1.7*

Open AccessPublished:March 06, 1998DOI:https://doi.org/10.1074/jbc.273.10.5851
      We report the isolation of a novel mouse voltage-gated Shaker-related K+ channel gene,Kv1.7 (Kcna7/KCNA7). Unlike other known Kv1 family genes that have intronless coding regions, the protein-coding region of Kv1.7 is interrupted by a 1.9-kilobase pair intron. The Kv1.7 gene and the related Kv3.3(Kcnc3/KCNC3) gene map to mouse chromosome 7 and human chromosome 19q13.3, a region that has been suggested to contain a diabetic susceptibility locus. The mouse Kv1.7 channel is voltage-dependent and rapidly inactivating, exhibits cumulative inactivation, and has a single channel conductance of 21 pS. It is potently blocked by noxiustoxin and stichodactylatoxin, and is insensitive to tetraethylammonium, kaliotoxin, and charybdotoxin. Northern blot analysis reveals ∼3-kilobase pair Kv1.7transcripts in mouse heart and skeletal muscle. In situhybridization demonstrates the presence of Kv1.7 in mouse pancreatic islet cells. Kv1.7 was also isolated from mouse brain and hamster insulinoma cells by polymerase chain reaction.
      Ion channels that exhibit a variety of gating patterns and ion selectivity are critical elements that transduce signals in diverse cell types (
      • Hille B.
      ). Voltage-gated potassium-selective (Kv)
      The abbreviations use are: Kv, voltage-gated potassium selective; PCR, polymerase chain reaction; RBL, rat basophilic leukemic; bp, base pair(s); mb, millibase pair(s); kb, kilobase pair(s).
      1The abbreviations use are: Kv, voltage-gated potassium selective; PCR, polymerase chain reaction; RBL, rat basophilic leukemic; bp, base pair(s); mb, millibase pair(s); kb, kilobase pair(s).
      channels represent the largest family within this class of proteins (
      • Chandy K.G.
      • Gutman G.A.
      ), and perform many vital functions in both electrically excitable and nonexcitable cells. Following initiation of an action potential in nerve and muscle cells, Kv channels play the important role of repolarizing the cell membrane (
      • Hille B.
      ). Kv channels can also modulate hormone secretion, for example insulin release from pancreatic islet cells (
      • Smith P.A.
      • Bokvist K.
      • Arkhammar P.
      • Berggren P.O.
      • Rorsman P.
      ,
      • Smith P.A.
      • Ashcroft F.M.
      • Rorsman P.
      ,
      • Philipson L.H.
      • Rosenberg M.
      • Kuznetsov A.
      • Lancaster M.E.
      • Worley III, J.F.
      • Roe M.W.
      • Dukes I.D.
      ,
      • Roe M.W.
      • Worley 3rd, J.F.
      • Mittal A.A.
      • Kuznetsov A.
      • DasGupta S.
      • Mertz R.J.
      • Witherspoon 3rd, S.M.
      • Blair N.
      • Lancaster M.E.
      • McIntyre M.S.
      • Shehee W.R.
      • Dukes I.D.
      • Philipson L.H.
      ), and regulate calcium signaling during mitogen-stimulated activation of lymphocytes (
      • Lewis R.S.
      • Cahalan M.D.
      ).
      Kv channels in mammalian cells are encoded by an extended family of at least nineteen genes (
      • Chandy K.G.
      • Gutman G.A.
      ). The largest subfamily, Kv1, is related to the fly Shaker gene and contains six members,Kv1.1–Kv1.6 (
      • Chandy K.G.
      • Gutman G.A.
      ). The Shaker gene has 21 exons, which can be alternatively spliced to generate at least five functionally distinct transcripts (
      • Pongs O.
      • Kecskemethy N.
      • Muller R.
      • Krah-Jentgens I.
      • Baumann A.
      • Kiltz H.H.
      • Canal I.
      • Llamazares S.
      • Ferrsu A.
      ,
      • Schwarz T.L.
      • Papazian D.M.
      • Caretto R.C.
      • Jan Y.N.
      • Jan L.Y.
      ). In contrast, the protein-coding regions of each of the six known mammalianKv1 genes and the three known Xenopus homologues are contained in a single exon (
      • Chandy K.G.
      • Gutman G.A.
      ,
      • Chandy K.G.
      • Williams C.B.
      • Spencer R.H.
      • Aguilar B.A.
      • Ghanshani S.
      • Tempel B.L.
      • Gutman G.A.
      ), precluding alternative splicing as a means of generating functionally different proteins. The evolutionary significance of this pattern of organization remains a puzzle.
      Here we report the identification of a novel mammalian gene,Kv1.7 (Kcna7/KCNA7), that has a genomic organization distinct from the other members of the vertebrateKv1 subfamily. We have defined the chromosomal location of this gene in the mouse and human genome, determined its tissue distribution, and characterized the biophysical and pharmacological properties of the cloned channel.

      RESULTS

      The Protein-coding Region of mKv1.7 Contains an Intron Unlike Its Vertebrate Homologues

      A restriction map of a 6.4-kb EcoRI DNA fragment containing the entire mouse Kv1.7 coding region is shown in Fig. 1. The coding region is contained in two exons separated by a 1.9-kb intron. The upstream exon encodes the entire N terminus, S1, and part of the S1-S2 loop. The downstream exon contains the region extending from the S1-S2 loop to the C-terminal end of the protein. The intron-exon splice sites were determined by comparing the genomic sequence with cDNA sequences obtained from the hamster insulinoma cell line, HIT-T1S, and from mouse brain (Fig. 1).
      The complete coding sequence of the mKv1.7 is shown in Fig. 2. The mKv1.7 sequence is identical in the N terminus from bp 52 to 996 with the mouse EST sequence AA021711. Betsholtz et al. (
      • Betsholtz C.
      • Baumann A.
      • Kenna S.
      • Ashcroft F.M.
      • Ashcroft S.J.
      • Berggren P.O.
      • Grupe A.
      • Pongs O.
      • Rorsman P.
      • Sandblom J.
      • Welsh M.
      ) amplified a short segment of Kv1.7 cDNA spanning the S5/S6 region from mouse (MK-6), rat (RK-6), and hamster (HaK-6) insulin-producing cells using PCR. Our sequence is identical to their MK-6 sequence, except for four nucleotide changes.
      Figure thumbnail gr2
      Figure 2Nucleotide sequence and deduced amino acid sequence of mouse Kv1.7. The six putative membrane-spanning domains (S1 through S6) and pore-forming region (P) are indicated. The potential sites for phosphorylation by tyrosine kinase (TY-K) and protein kinase C (PKC) are shown. The position of the single intron between S1 and S2 is indicated by an arrow, and the critical residue for tetraethylammonium block (Ala-441, within the P region) is shown in bold and underlined.
      The deduced mKv1.7 protein consists of 532 amino acids and contains six putative membrane-spanning domains, S1–S6 (Fig. 2). The hydrophobic core of this protein shares considerable sequence similarity with otherShaker family channels, while the intracellular N and C termini and the external loops between S1/S2 and S3/S4 show little conservation. The protein contains conserved sites for post-translational modifications as indicated in Fig. 2. As do all other Shaker-related channels, mKv1.7 has a potential tyrosine kinase phosphorylation site (RPSFDAVLY) in its N-terminal region (
      • Chandy K.G.
      • Gutman G.A.
      ); the proline-rich stretch within the N terminus (see residues 29–42) may be a binding site for SH3 domains of tyrosine kinases. Two protein kinase C consensus sites (Ser/Thr-X-Arg/Lys) are present in the cytoplasmic loop between S4 and S5 of mKv1.7; at least one of these sites is present in all members of the Kv1 family (
      • Chandy K.G.
      • Gutman G.A.
      ). mKv1.7, like Kv1.6, lacks anN-glycosylation site in the extracellular loop linking the S1 and S2 transmembrane segments; this consensus sequence is conserved in all other Kv1 family genes.
      Fig. 3 shows a phylogenetic tree of the entire Shaker family of genes based on parsimony analysis of a nucleotide sequence alignment (generated from the amino acid sequence alignment) using the program PAUP (
      • Swofford D.L.
      ). Our analysis placesmKv1.7 within the Shaker family of genes. The mKv1.7 gene does not cluster with any of the known genes, and its position within the tree is not firmly established.
      Figure thumbnail gr3
      Figure 3Proposed phylogenetic relationship of mKv1.7 and other Shaker-related Kvchannel genes. This tree is based on parsimony analysis of nucleotide sequence alignments using the program PAUP (
      • Swofford D.L.
      ). Horizontal distance represents the number of nucleotide substitutions in each lineage, with the scale bar at the upper leftrepresenting 100 substitutions. m, mouse; h, human; x, Xenopus; APLK,Aplysia; SHAKER, Drosophila Shaker.

      Kv1.7 Is Located on Mouse Chromosome 7 and Human Chromosome 19q13.3

      The mKv1.7/Kcna7 gene resides on mouse chromosome 7 (Fig. 4 A), ∼0.5 centimorgan telomeric to the Shaw-related K+ channel gene,mKv3.3/Kcnc3, and ∼2.4 centimorgans centromeric of MyoD1 (myoblast differentiation factor). The most centromeric marker in this study was Gpi1 (glucose phosphate isomerase 1), which mapped ∼6.1 centimorgans centromeric tomKv3.3/Kcnc3.
      Figure thumbnail gr4
      Figure 4Chromosomal localization of the mouse and human Kv1.7 genes. A, mouse chromosome 7. (Left) results of segregation analysis in a (C57BL/6J × M. spretus)F1 × C57BL/6J interspecific backcross. The genes indicated are as follows: Gpi1, glucose phosphate isomerase-1; Fcrn, Fc-receptor, neonatal form;Myod1, myoblast differentiation factor-1. The ratios of the total number of mice exhibiting recombinant chromosomes to the total number of mice analyzed for each pair of loci are Gpi1 - 11/181 - Fcrn - 0/184 - mKv3.3/Kcnc3 - 1/188 -mKv1.7/Kcna7 - 4/167 - Myod1. The recombination frequencies expressed as genetic distances in centimorgans ± the S.E are: Gpi1 - 6.1 ± 1.8 - [Fcrn,mKv3.3] - 0.5 ± 0.5 - mKv1.7 - 2.4 ± 1.2 - MyoD1. Filled boxes indicate the presence of the C57Bl/6J allele, and open boxes the presence of the M. spretus allele. (Right) diagram showing the deduced order of Kv1.7 and neighboring genes on mouse chromosome 7, with the centromere shown at the top. B, human chromosome 19. Diagram shows the deduced order of genes on human chromosome 19. A single hKv1.7/KCNA7 cosmid was mapped to 19q13.3 by fluorescent in situ hybridization (FISH). Ten cells were scored for each cosmid used, and for each of twohKv3.3/KCNC3 cosmids, signal was present on both chromatids in a position corresponding to 19q13.3-q13.4. The positions where signal was observed for the two probes are indicated as open circles (hKv1.7/KCNA7) and solid circles(hKv3.3/KCNC3). C, detailed map of the relevant region of human chromosome 19. The positions of KCNA7,KCNC3, GYS1, and HRC are shown, with each cross-bar indicating a distance of 100 kb. The positions corresponding to 50.5 and 52 mb of chromosome 19 (
      • Ashworth L.K.
      • Batzer M.A.
      • Brandriff B.
      • Branscomb E.
      • de Jong P.
      • Garcia E.
      • Garnes J.A.
      • Gordon L.A.
      • Lamerdin J.E.
      • Lennon G.
      • Mohrenweiser H.
      • Olsen A.S.
      • Slezak T.
      • Carrano A.V.
      ) are indicated, as is the point of demarcation between chromosome bands q13.3 and q13.4.
      The interval on mouse chromosome 7 containing mKv1.7/Kcna7 and mKv3.3/Kcnc3 shares a region of homology with human chromosomes 19q13 and 11p15, and the human homologues of these K+ channel genes may therefore be expected to reside on one of these chromosomes. Confirming this prediction, we mapped both genes to human 19q13.3–13.4 using fluorescent in situhybridization. The idiogram of human chromosome 19 shown in Fig. 4 B, and a more detailed map shown in Fig. 4 C, indicate that hKv1.7/KCNA7 is located ∼1.3 mb centromeric of hKv3.3/KCNC3. The genes for both muscle glycogen synthase (GYS1) and the histidine-rich calcium protein (HRC) also map to this region; Kv1.7/KCNA7 lies ∼25 kb telomeric to GYS1 and ∼200 kb centromeric to HRC (Fig. 4 C). Interestingly, a putative diabetes susceptibility gene has been suggested to be present at 19q13.3 (
      • Groop L.C.
      • Kankuri M.
      • Schalin-Jantti C.
      • Ekstrand A.
      • Nikula-Ijas P.
      • Widen E.
      • Kuismanen E.
      • Eriksson J.
      • Franssila-Kallunki A.
      • Saloranta C.
      • Koskimies S.
      ,
      • Elbein S.C.
      • Hoffman M.
      • Ridinger D.
      • Otterud B.
      • Leppert M.
      ), especially in Finnish families with associated hypertension and difficulties in insulin-stimulated glucose storage. This region has also been suggested to contain a modifier locus for cystic fibrosis (
      • Estivill X.
      ).

      Biophysical and Pharmacological Characterization of Kv1.7 Channels

      We carried out a detailed characterization of mKv1.7 channels expressed in RBL cells which express no native Kv currents (
      • McCloskey M.
      • Cahalan M.D.
      ,
      • Nguyen QA
      • Kath J.
      • Hanson D.C.
      • Biggers M.S.
      • Canniff P.C.
      • Donovan C.B.
      • Mather R.J.
      • Bruns M.J.
      • Rauer H.
      • Aiyar J.
      • Lepple-Wienhues A.
      • Gutman G.A.
      • Grissmer S.
      • Cahalan M.D.
      • Chandy K.G.
      ). The mKv1.7 gene expressed in Xenopus oocytes produced currents (data not shown) similar to those obtained in RBL cells (Fig. 5).
      Figure thumbnail gr5
      Figure 5Kv1.7 currents. A, family of mKv1.7 currents. The holding potential was −80 mV and depolarizing pulses were applied every 30 s. The test potential was changed from −50 to 50 mV in 10-mV increments. B, peak K+ conductance-voltage relation for currents shown in A. The line through the points was fitted with the Boltzmann equation: gk (E) =gk(max)/{1 + exp[(EnE)/k]}, with parameter values gk(max) = 20 nS and k = −8 mV. C, cumulative inactivation of Kv1.7 currents. Currents were elicited by a train of six depolarizing voltage steps (200-ms duration) to 40 mV once every second from a holding potential of −80 mV. The current amplitude decreases significantly during this train of pulses from the largest (first trace) to the smallest (last). D, kinetics of deactivation of Kv1.7 currents. Tail currents were elicited by voltage steps from −100 to −40 mV after a 15-ms depolarizing prepulse to 40 mV. The tail current-decay time constants, τt, were measured by fitting single-exponential functions to the decay of the K+ current during repolarization. E, single-channel currents of Kv1.7 in an outside-out patch. The broken line shows the slope conductance.

      Voltage Dependence

      Patch clamp studies were performed in the whole-cell mode. Fig. 5 A shows a family of outward currents elicited by a 200 ms depolarizing pulse from a holding potential of −80 mV in RBL cells injected with mKv1.7 cRNA; no outward currents were detected in control cells (data not shown). The currents activate rapidly, and from the conductance-voltage curve shown in Fig. 5 B we determined that the 1/2 activation potential (V1/2) is −20 mV.

      Inactivation and Deactivation

      Inactivation of mKv1.7 channels was rapid following a 200 ms depolarizing pulse from −80 to 40 mV (Fig. 5 A). The rate of inactivation, measured by fitting the data to a single exponential function, yielded a time constant (τh) of 14 ± 2.1 ms (S.E., n = 7). As shown in Fig. 5 C, the current became progressively smaller following repeated depolarizing pulses at 1-s intervals. This phenomenon, termed “cumulative inactivation,” is due to the accumulation of channels in the inactivated state which are then unavailable for opening. The related channels, Kv1.3 (
      • Lewis R.S.
      • Cahalan M.D.
      ) and Kv1.4 (
      • Wymore R.
      • Korenberg J.R.
      • Coyne C.
      • Chen X-N
      • Hustad C.
      • Copeland N.G.
      • Gutman G.A.
      • Jenkins N.A.
      • Chandy K.G.
      ), also display this behavior.
      The kinetics of channel closing (deactivation) was determined by first opening the channels with a 15 ms conditioning depolarizing pulse, and then forcing the channels to close by repolarizing to different potentials (Fig. 5 D). The time constant, τtail, of the resulting “tail” current was 5.1 and 5.3 ms at −60 mV in two cells.

      Single-channel Conductance

      We measured single-channel currents in an outside-out patch by applying 450-ms voltage ramps from −90 to 80 mV every second (Fig. 5 E). Single channel events were seen at potentials more positive than ∼−15 mV. The single-channel conductance of 21 pS was estimated from the slope of the current recorded during an opening (Fig. 5 E).

      Pharmacology

      We determined the pharmacological sensitivity of the mKv1.7 channel using methods described previously (
      • Nguyen QA
      • Kath J.
      • Hanson D.C.
      • Biggers M.S.
      • Canniff P.C.
      • Donovan C.B.
      • Mather R.J.
      • Bruns M.J.
      • Rauer H.
      • Aiyar J.
      • Lepple-Wienhues A.
      • Gutman G.A.
      • Grissmer S.
      • Cahalan M.D.
      • Chandy K.G.
      ,
      • Grissmer S.
      • Nguyen A.N.
      • Aiyar J.
      • Hanson D.C.
      • Mather R.J.
      • Gutman G.A.
      • Karmilowicz M.J.
      • Auperin D.D.
      • Chandy K.G.
      ), IC50 values in each case being determined when block reached steady-state. The channel was blocked by several non-peptide small molecule antagonists, 4-aminopyridine (IC50 = 245 μm), capsaicin (25 μm), cromakalim (450 μm), tedisamil (18 μm), nifedipine (13 μm), diltiazem (65 μm), and resiniferatoxin (20 μm). Surprisingly, the dihydroquinoline compound, CP-339,818, that blocks rapidly inactivating Kv1 channels in the nanomolar range (
      • Nguyen QA
      • Kath J.
      • Hanson D.C.
      • Biggers M.S.
      • Canniff P.C.
      • Donovan C.B.
      • Mather R.J.
      • Bruns M.J.
      • Rauer H.
      • Aiyar J.
      • Lepple-Wienhues A.
      • Gutman G.A.
      • Grissmer S.
      • Cahalan M.D.
      • Chandy K.G.
      ), had little effect on mKv1.7 (IC50 = 10 μm). The channel was insensitive to externally applied tetraethylammonium (C50 = 86 mm), probably because the residue at the tetraethylammonium-binding site, Ala-441 (Fig. 2), is hydrophobic.
      The mKv1.7 channel is also potently blocked by a peptide (ShK toxin) obtained from sea anemone Stichodactyla helianthus(IC50 = 13 nm), and by the scorpion toxins, noxiustoxin (IC50 = 18 nm) and margatoxin (IC50 = 116 nm). The channel was resistant to charybdotoxin (IC50 >1000 nm) and kaliotoxin (IC50 >1000 nm).

      Expression of mKv1.7 Transcripts in Different Tissues

      Northern blot assays using a mKv1.7-specific probe revealed strongly hybridizing 3-kb bands in heart and skeletal muscle; faint bands of similar size were visible in liver and lung (together with larger 7–8-kb bands), but none were seen in spleen, kidney, testis, or brain (Fig. 6) We were able to isolatemKv1.7 transcripts from mouse brain by PCR (see Fig. 1). mKv1.7 is also expressed in placenta, since the mouse EST AA021711 was derived from this tissue.
      Figure thumbnail gr6
      Figure 6Expression of Kv1.7 mRNA in tissues. Northern blot assay.
      PCR analysis demonstrated the presence of haKv1.7 mRNAs in hamster insulinoma cells (Fig. 1). We verified the presence of mKv1.7 mRNA in pancreatic islet cells obtained from 9–16-week-old diabetic db/db mice by in situhybridization (Fig. 7 C) using a mKv1.7-specific antisense probe (
      • deJong P.J.
      • Yokabata K.
      • Chen C.
      • Lohman F.
      • Pederson L.
      • McNinch J.
      • van Dilla M.
      ,
      • Permutt M.A.
      • Koranyi L.
      • Keller K.
      • Lacy P.E.
      • Scharp D.W.
      • Mueckler M.
      ,
      • Chen H.
      • Charlat O.
      • Tartaglia L.A.
      • Woolf E.A.
      • Weng X.
      • Ellis S.J.
      • Lakey N.D.
      • Culpepper J.
      • Moore K.J.
      • Breitbart R.E.
      • Duyk G.M.
      • Tepper R.
      • Morgenstern J.P.
      ); mKv1.7 mRNA was also present in islets from normal db/+ mice (data not shown). Scattered acinar cells outside the islets also showed mKv1.7 hybridization (Fig. 7 C). In contrast, mKv3.4mRNA was found in acinar cells surrounding islets, but not in islets, of both db/db (Fig. 7B) and db/+ mice (data not shown). As a control, insulin mRNA was detected in both normal and diabetic islets, but not in acinar cells (Fig. 7 A). A Kv1.5-specific probe did not show appreciable hybridization to islets (data not shown), despite a report of Kv1.5 cDNA having been cloned from human insulinoma cells (
      • Philipson L.H.
      • Hice R.E.
      • Schaefer K.
      • LaMendola J.
      • Bell G.I.
      • Nelson D.J.
      • Steiner D.F.
      ).
      Figure thumbnail gr7
      Figure 7In situ hybridization of mouse pancreas from diabetic db/db mice showing expression of Kv1.7, Kv3.4, and insulin. A–C, pancreatic sections from a db/db mouse hybridized with probes specific for insulin (A), Kv3.4 (B), or Kv1.7 (C). Top, sense probe, dark field;middle, antisense probe, dark field; bottom, antisense probe, bright field, showing the same field as the middle row. Filled arrow, pancreatic islet; open arrow, acinar cells that hybridized with Kv1.7 antisense probe.A, sense and antisense probes, 0.1 ng/ml, 10 days of exposure; B, sense probe, 0.1 ng/μl, 10 days of exposure, and antisense probe, 0.5 ng/μl, 7 days of exposure; C, sense and antisense probes, 0.5 ng/μl, 1 month of exposure. Magnification: A and B, × 425; C, × 312.

      DISCUSSION

      Unlike all other known mammalian Shaker-related genes (Kv1.1–Kv1.6) that have intronless coding regions (
      • Chandy K.G.
      • Gutman G.A.
      ,
      • Schwarz T.L.
      • Papazian D.M.
      • Caretto R.C.
      • Jan Y.N.
      • Jan L.Y.
      ), the protein-coding region of mKv1.7 is interrupted by a single 1.9-kb intron. The fly Shaker gene also contains an intron in the S1-S2 loop, raising the possibility that the intron in Kv1.7 may be ancient, predating the divergence of flies and mammals. Both the mouse Kv.1.7 and the fly Shaker intron are placed between codons, i.e. they are “phase 0” introns. While this is consistent with their having a common origin it may also be fortuitous, since there are only three possible “phases.” Although we favor the idea that Kv introns were lost in the vertebrate lineage before their expansion by gene duplication (in which case the Kv1.7 intron would represent a more recent insertion), the evolutionary history of this complex gene family remains to be elucidated.
      Since Kv1.7 mRNA is expressed in the mouse heart, we searched the literature for native cardiac A-type Kv currents with properties resembling those of Kv1.7. The Kv1.7 homotetramer shares many properties with the rapidly inactivating transient outward (Ito) current in cardiac Purkinje fibers, but not the Ito current in atrial and ventricular myocytes. Kv1.7 and the Purkinje Ito currents activate at negative potentials (∼−30 to −20 mV), inactivate rapidly (τh < 25 ms), exhibit cumulative inactivation, are blocked by micromolar concentrations of 4-aminopyridine, and are resistant to >20 mm tetraethylammonium (
      • Reder R.F.
      • Miura D.S.
      • Danilo Jr., P.
      • Rosen M.R.
      ,
      • Gintant G.A.
      • Cohen I.S.
      • Datyner N.B.
      • Kline R.P.
      ,
      • Dixon J.E.
      • Shi W.
      • Wang H.S.
      • McDonald C.
      • Yu H.
      • Wymore R.S.
      • Cohen I.S.
      • McKinnon D.
      ) (this study). In contrast, the Ito current in atrial and ventricular muscle, a product of the Kv4.3 gene, does not exhibit cumulative inactivation (
      • Dixon J.E.
      • Shi W.
      • Wang H.S.
      • McDonald C.
      • Yu H.
      • Wymore R.S.
      • Cohen I.S.
      • McKinnon D.
      ). These studies suggest that at least part of the Purkinje fiber Ito might be encoded by the Kv1.7 gene, although more extensive biophysical and pharmacological studies are required to confirm the link, and the presence of Kv1.7 mRNA and/or protein has yet to be demonstrated in these fibers. The abundant expression of Kv1.7 mRNA in mouse heart suggests that this channel is also likely to be present in ventricular and/or atrial muscle where it may co-assemble with other Kv1 family channels to form heterotetramers.
      Recent studies suggest an important role for Kv channels in regulating islet cell function, specifically in repolarizing the membrane potential following each action potential during the glucose-induced bursting phase associated with insulin secretion (
      • Smith P.A.
      • Bokvist K.
      • Arkhammar P.
      • Berggren P.O.
      • Rorsman P.
      ,
      • Smith P.A.
      • Ashcroft F.M.
      • Rorsman P.
      ,
      • Philipson L.H.
      • Rosenberg M.
      • Kuznetsov A.
      • Lancaster M.E.
      • Worley III, J.F.
      • Roe M.W.
      • Dukes I.D.
      ,
      • Roe M.W.
      • Worley 3rd, J.F.
      • Mittal A.A.
      • Kuznetsov A.
      • DasGupta S.
      • Mertz R.J.
      • Witherspoon 3rd, S.M.
      • Blair N.
      • Lancaster M.E.
      • McIntyre M.S.
      • Shehee W.R.
      • Dukes I.D.
      • Philipson L.H.
      ). Despite these interesting findings, the genes encoding Kv genes in β-cells have not been identified. Although the Kv1.5 gene was isolated from human insulinoma cells (
      • Philipson L.H.
      • Hice R.E.
      • Schaefer K.
      • LaMendola J.
      • Bell G.I.
      • Nelson D.J.
      • Steiner D.F.
      ), we did not detectKv1.5 mRNA in normal or diseased islets. We have, however, demonstrated the presence of Kv1.7 mRNA in these cells. Unlike the noninactivating Kv channels in pancreatic β-cells (
      • Smith P.A.
      • Bokvist K.
      • Arkhammar P.
      • Berggren P.O.
      • Rorsman P.
      ,
      • Smith P.A.
      • Ashcroft F.M.
      • Rorsman P.
      ), the Kv1.7 homotetramer exhibits rapid C-type inactivation. Since Kv1.7 mRNA is expressed in pancreatic islets, it is possible that heteromultimers composed of Kv1.7 and other Kv1 subunits constitute the native Kv channels in β-cells. Enhanced levels of such Kv channels would excessively hyperpolarize the membrane of β-cells and impair insulin secretion (
      • Philipson L.H.
      • Rosenberg M.
      • Kuznetsov A.
      • Lancaster M.E.
      • Worley III, J.F.
      • Roe M.W.
      • Dukes I.D.
      ). The mapping of the Kv1.7 gene to human chromosome 19q13.3, a region thought to contain a diabetic susceptibility gene (
      • Groop L.C.
      • Kankuri M.
      • Schalin-Jantti C.
      • Ekstrand A.
      • Nikula-Ijas P.
      • Widen E.
      • Kuismanen E.
      • Eriksson J.
      • Franssila-Kallunki A.
      • Saloranta C.
      • Koskimies S.
      ,
      • Elbein S.C.
      • Hoffman M.
      • Ridinger D.
      • Otterud B.
      • Leppert M.
      ), also suggests that Kv1.7 might contribute to the pathogenesis of type II diabetes mellitus in some humans.
      In conclusion, we have described a novel Kv1 family gene with a genomic organization distinct from all the other members of the family. The Kv1.7 channel produces a typical A-type current, and transcripts are expressed in the heart, skeletal muscle, brain, placenta, and pancreatic β-cells. This channel is biophysically and pharmacologically similar to the Purkinje fiber Itocurrent, and the gene may contribute at least one subunit to heteromultimeric Kv channels in pancreatic β-cells.

      Acknowledgments

      The assistance of F. Glaser, S. Plummer, B. Dethlefs, S. Hoffman, M. Christensen, T. Wymore, C. Chandy, and D. J. Gilbert is gratefully acknowledged. We are obliged to Dr. J. Aiyar for reading and improving our manuscript.

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