Changes in cAMP Affinity Modulate HCN Channel Activity

Abstract

Wang et al. describe a mechanism whereby activity-dependent changes in adenosine 3′,5′-monophosphate (cAMP) binding affinity, rather than changes in cAMP concentration, can modulate activity of the hyperpolarization-activated, cyclic nucleotide-gated, cation nonselective (HCN) channel and can help regulate rhythmic firing. HCN channels are involved in pace-making and are modulated by cAMP, which facilitates channel opening. This has been modeled as an allosteric interaction in which channel opening is coupled with a transition to a conformation that enhances cAMP binding. Wang et al. used inside-out patch-clamp and voltage-clamp analyses of oocytes expressing HCN2 channel mRNA to test this model. Concentrations of cAMP ≥10 μM accelerated current activation and shifted the voltage dependence to more positive potentials, whereas 10 nM cAMP produced a slow phase of activation, consistent with preferential binding of cAMP to the open channel, shifting the equilibrium between closed and open unbound channels and eliciting a delayed phase of channel opening. Activation kinetics of channels in inside-out patches without cAMP fit a single exponential time course; a second exponential was apparent when cAMP was present. Activation kinetics of channels in intact oocytes resembled those of cell-free patches in low concentrations of cAMP; mutant channels that were unable to bind cAMP, however, showed only a single exponential component. Normal--but not mutant--channels showed "memory" and responded to a train of hyperpolarizations with a prolonged tail of channel opening that lasted for tens of seconds. Modeling studies indicated that cAMP modulation of neuronal HCN channels could play an important role in regulating rhythmic activity. J. Wang, S. Chan, M. F. Nolan, S. A. Siegelbaum, Activity-dependent regulation of HCN pacemaker channels by cyclic AMP: Signaling through dynamic allosteric coupling. Neuron 36 , 451-461 (2002). [Online Journal]