Regulation of HCN Channels by Protein Interactions

Abstract

Hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels are key regulators of subthreshold membrane potentials in excitable cells. The four mammalian HCN channel isoforms, HCN1-HCN4, are expressed throughout the body, where they contribute to diverse physiological processes including cardiac pacemaking, sleep-wakefulness cycles, memory, and somatic sensation. While all HCN channel isoforms produce currents when expressed by themselves, an emerging list of interacting proteins shape HCN channel excitability to influence the physiologically relevant output. The best studied of these regulatory proteins is the auxiliary subunit, TRIP8b, which binds to multiple sites in the C-terminus of the HCN channels to regulate expression and disrupt cAMP binding to fine-tune neuronal HCN channel excitability. Less is known about the mechanisms of action of other HCN channel interaction partners like filamin A, Src tyrosine kinase, and MinK-related peptides, which have a range of effects on HCN channel gating and expression. More recently, the inositol trisphosphate receptor-associated cGMP-kinase substrates IRAG1 and LRMP (also known as IRAG2), were discovered as specific regulators of the HCN4 isoform. This review summarizes the known protein interaction partners of HCN channels and their mechanisms of action and identifies gaps in our knowledge.

Keywords: HCN channels; IRAG; KCNE; TRIP8b; accessory proteins; protein subunits.

FIGURE 1

HCN channel structure, currents and regulation by cAMP. (A) CryoEM structure of HCN4 showing only two of the four subunits for clarity (PDB ID:6YGO). The six transmembrane-spanning domains in each subunit form the voltage sensing domain (S1-S4, blue) and the pore domain (S5-S6, green). The C-linker (yellow) and cyclic nucleotide binding domain (CNBD, orange) in the C-terminus mediate the response to cyclic nucleotides. (B) Conductance-voltage relationships for HCN4 channels in the absence (black) or presence (red) of cAMP. Hyperpolarization-activated currents recorded from a HEK cell expressing HCN4 before (black) and after (red) perfusion of 100 µM cAMP (inset).

FIGURE 2

Schematic illustration of TRIP8b and its interaction with HCN channels. The core domain of TRIP8b (yellow) antagonizes cAMP binding to the CNBD of HCN channels (blue). The TRP domains (green) interact with the SNL motif in the distal C-terminus to regulate HCN channel expression. From DeBerg et al., 2015.

FIGURE 3

LRMP and IRAG are isoform-specific regulators of HCN4. (A) Domain structure of LRMP and IRAG. (B) LRMP inhibits cAMP-dependent regulation of HCN4. (C) IRAG acts a cAMP mimetic that potentiates HCN4 in the absence of cAMP. From Peters et al., 2020a.