HCN Channels : the Molecular Basis for their cAMP-TRIP8b Regulation

Hyperpolarization-activated cyclic nucleotide-regulated (HCN1-4) channels are involved in the regulation of several higher order neural functions, such as short- and long-term memory processes (1). HCN channels are exquisitely sensitive to endogenous levels of cAMP, since they directly bind cAMP through a specialized domain in their cytoplasmic C-terminus (cyclic nucleotide binding domain, CNBD) (2). In addition to cAMP, HCN channels are further regulated by TRIP8b, their brain-specific auxiliary subunit. TRIP8b antagonizes the effect of cAMP on HCN channel opening, as it interacts with the CNBD of the channel (3). We employed solution NMR methodologies to determine the 3D structure of the human HCN2 CNBD in the cAMP-free form, and mapped on it the TRIP8b interaction site. Thus, we were able to reconstruct, for the first time, the molecular mechanisms underlying the dual regulation of HCN channel activity by cAMP-TRIP8b (4). Furthermore, site-directed mutagenesis followed by biochemical/biophysical analysis allowed us to identify key residues within the CNBD involved in TRIP8b binding. These new structural information will provide deeper insights into the molecular basis of neurological disorders associated with dysfunction of the HCN channel conductance in neurons, potentially leading to the design of drugs able to modulate HCN channel mediated memory processes.