Control of Pacemaker channel function by cyclic AMP.
We currently focus on the pacemaker ion channel, which forms a pore through which potassium and sodium ions flow. In the natural pacemaker of the heart, the flow of ions through this channel helps the cells to contract and beat autonomously; hence the name “pacemaker” ion channel. Pacemaker channels contribute to changes in heartbeat that occur in response to hormones such as adrenaline, which is released during the “fight or flight” response to a stressor. Adrenaline triggers the production of a small molecule, cyclic (c)AMP, inside the pacemaker cell which then attaches itself to the channel, allowing it to open more easily.
Greater electrical activity and a faster beating rate result, permitting an adaptive, heightened physical reaction to the stressor. Because they are opened by hyperpolarization of the membrane potential, and opening is facillitated by cAMP, pacemaker channels are also referred to as Hyperpolarization-activated Cyclic Nucleotide-gated or HCN channels. HCN channels are made up of four individual subunits that assemble within the cell to produce the pacemaker ion channel. Thus, each channel has four cAMP binding sites facing the internal side of cells.
In our lab, we are interested in understanding how binding of cAMP ultimately leads to facilitation of pacemaker channel opening. Below is a figure from our paper Chow et al, 2012, which shows negatively cooperative binding of cAMP to a tetrameric version of the HCN2 C-terminus.
Cyclic AMP binds to a tetrameric version of the HCN2 C-terminus with an apparent high and low affinity. A, plots of heat produced by ITC upon progressive injections of cAMP into 200 μM HCN2 C-terminus protein containing the cAMP binding domain and the C-linker, which attaches the binding domain to the pore in the full-length channel. In the upper plot, the broadening of peaks causes an initial increase in the overall heat per injection. The solid line through the values of the lower plot represents a two independent binding site model, which yielded values for affinity and energetics (ΔG, ΔH, and ΔS). B, bar graph comparing high and low cAMP binding affinity as determined from the fit in A. Error bars indicate S.D. C, bar graph comparing energetics between the low and high affinity binding events, determined from the fit in A. Error bars indicate S.D. D, plots of heat produced by ITC upon progressive injections of cAMP into 200 μM HCN2 protein.