Cation-pi interactions as a possible mechanism for controlling the closing of Hyperpolarization-activated cyclic nucleotide-modulated ion channels

The hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channel gene family is known to contribute significantly to cardiac pacemaking via pacemaker currents. To date, there are four mammalian HCN isoforms (HCN1–4) identified. The importance of HCN channel function to normal cardiac automaticity in mice was recently corroborated in humans diagnosed with idiopathic sinus node dysfunction. While the importance of HCN channels is widely recognized amongst cardiologists and scientists of the field, the specific mechanism of HCN channel opening or closing remain elusive. It is known that HCN channels are activated via hyperpolarization, and it is recently discovered that there is a voltage-independent step in the opening or closing of the channel (and at the same time a rate-limiting step). One of the leading theories for HCN channel closing is the cation-pi interaction between Phenylalanine and a positively charged component in the channel. In this proposal, we introduce some basic HCN structures and channel dynamics, then outline the basic principles of measuring current traces with patch clamping. Finally, we characterize the use of new constructs of mutant HCN channels expressed in Xenopus laevis (i.e., frog) oocytes. We also present a four-state cyclic allosteric scheme in the absence of cAMP for the mechanism in regulating HCN channel opening and closing. Our goal is to compare and contrast mutant HCN channel current traces with wild type HCN channels in order to gain a better understanding of HCN regulation of currents and ultimately suggest a fitting model for its mechanisms.