title: Cellular context and multiple channel domains determine cAMP sensitivity of HCN4 channels: ligand-independent relief of autoinhibition in HCN4.
abstract: Hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels produce the I(f) and I(h) currents, which are critical for cardiac pacemaking and neuronal excitability, respectively. HCN channels are modulated by cyclic AMP (cAMP), which binds to a conserved cyclic nucleotide-binding domain (CNBD) in the C terminus. The unliganded CNBD has been shown to inhibit voltage-dependent gating of HCNs, and cAMP binding relieves this "autoinhibition," causing a depolarizing shift in the voltage dependence of activation. Here we report that relief of autoinhibition can occur in the absence of cAMP in a cellular context- and isoform-dependent manner: when the HCN4 isoform was expressed in Chinese hamster ovary (CHO) cells, the basal voltage dependence was already shifted to more depolarized potentials and cAMP had no further effect on channel activation. This "pre-relief" of autoinhibition was specific both to HCN4 and to CHO cells; cAMP shifted the voltage dependence of HCN2 in CHO cells and of HCN4 in human embryonic kidney (HEK) cells. The pre-relief phenotype did not result from different concentrations of soluble intracellular factors in CHO and HEK cells, as it persisted in excised cell-free patches. Likewise, it did not arise from a failure of cAMP to bind to the CNBD of HCN4 in CHOs, as indicated by cAMP-dependent slowing of deactivation. Instead, a unique ∼300-amino acid region of the distal C terminus of HCN4 (residues 719-1012, downstream of the CNBD) was found to be necessary, but not sufficient, for the depolarized basal voltage dependence and cAMP insensitivity of HCN4 in CHO cells. Collectively, these data suggest a model in which multiple HCN4 channel domains conspire with membrane-associated intracellular factors in CHO cells to relieve autoinhibition in HCN4 channels in the absence of cAMP. These findings raise the possibility that such ligand-independent regulation could tune the activity of HCN channels and other CNBD-containing proteins in many physiological systems.
authors.affiliation: Department of Physiology and Biophysics, University of Colorado, Anschutz Medical Campus, Aurora, CO 80045, USA.
authors.forename: Zhandi
authors.initials: Z
authors.lastname: Liao
authors.forename: Dean
authors.initials: D
authors.lastname: Lockhead
authors.forename: Joshua R
authors.initials: JR
authors.lastname: St Clair
authors.forename: Eric D
authors.initials: ED
authors.lastname: Larson
authors.forename: Courtney E
authors.initials: CE
authors.lastname: Wilson
authors.forename: Catherine
authors.initials: C
authors.lastname: Proenza
mesh_terms: D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D000242:Cyclic AMP / Q000378:metabolism* / Q000494:pharmacology; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D057809:HEK293 Cells; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015640:Ion Channel Gating / Q000187:drug effects* / Q000502:physiology; D007473:Ion Channels / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D008024:Ligands; D008564:Membrane Potentials / Q000187:drug effects; D015221:Potassium Channels; D020033:Protein Isoforms / Q000502:physiology; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000502:physiology
chemical_list: D054815:Cyclic Nucleotide-Gated Cation Channels; C578710:HCN2 protein, human; C578711:Hcn2 protein, mouse; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D008024:Ligands; D015221:Potassium Channels; D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins; D000242:Cyclic AMP