Functional expression of the human HCN3 channel

Abstract

Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels underlie the inward pacemaker current, termed If/Ih, 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 V1\2 was -77 ± 5.4 m V, and the reversal potential Erev was -20.5 ± 4 mV, resulting in a permeability ratio PNa/PK 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. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.