Functional expression of the human HCN3 channel.
Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels underlie the inward pacemaker current, termed I(f)/I(h), in a variety of tissues. Many details are known for the HCN subtypes 1, 2, and 4. We now successfully cloned the cDNA for HCN3 from human brain and compared the electrophysiological properties of hHCN3 to the other three HCN subtypes. Overexpression of human HCN3 channels in HEK293 cells resulted in a functional channel protein. Similar to hHCN2 channels, hHCN3 channels are activated with a rather slow time constant of 1244 +/- 526 ms at -100 mV, which is a greater time constant than that of HCN1 but a smaller one than that of HCN4 channels. The membrane potential for half-maximal activation V((1/2)) was -77 +/- 5.4 mV, and the reversal potential E(rev) was -20.5 +/- 4 mV, resulting in a permeability ratio P(Na)/P(K) of 0.3. Like all other HCNs, hHCN3 was inhibited rapidly and reversibly by extracellular cesium and slowly and irreversibly by extracellular applied ZD7288. Surprisingly, the human HCN3 channel was not modulated by intracellular cAMP, a hallmark of the other known HCN channels. Sequence comparison revealed >80% homology of the transmembrane segments, the pore region, and the cyclic nucleotide binding domain of hHCN3 with the other HCN channels. The missing response to cAMP distinguishes human HCN3 from both the well cAMP responding HCN subtypes 2 and 4 and the weak responding subtype 1.
Stieber
Juliane
J
Institut für Pharmakologie und Toxikologie der Technischen Universität München, Biedersteiner Strasse 29, 80802 München, Germany. stieber@ipt.med.tu-muenchen.de
Stöckl
Georg
G
Herrmann
Stefan
S
Hassfurth
Benjamin
B
Hofmann
Franz
F
D015152:Blotting, Northern; D015153:Blotting, Western; D001921:Brain / Q000378:metabolism; D002316:Cardiotonic Agents / Q000494:pharmacology; D002412:Cations; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002586:Cesium / Q000737:chemistry / Q000494:pharmacology; D003001:Cloning, Molecular; D000242:Cyclic AMP / Q000378:metabolism; D054815:Cyclic Nucleotide-Gated Cation Channels; D018076:DNA, Complementary / Q000378:metabolism; D004594:Electrophysiology; D005786:Gene Expression Regulation; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels / Q000737:chemistry; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D010957:Plasmids / Q000378:metabolism; D015221:Potassium Channels / Q000096:biosynthesis* / Q000502:physiology*; D011743:Pyrimidines / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D017422:Sequence Analysis, DNA; D013997:Time Factors; D014162:Transfection
D002316:Cardiotonic Agents; D002412:Cations; D054815:Cyclic Nucleotide-Gated Cation Channels; D018076:DNA, Complementary; C472018:HCN3 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D015221:Potassium Channels; D011743:Pyrimidines; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; C082246:ICI D2788; D002586:Cesium; D000242:Cyclic AMP