Allostery modulates the beat rate of a cardiac pacemaker

Abstract

Edited by Norma AllewellThe S672R mutation in heart cell ion channels leads to low heart rates and arrhythmia by an unknown route. A multifaceted NMR analysis now demonstrates that this mutant impacts allosteric coupling in domains inside of the cell to change channel activation, providing a mechanistic explanation for phenotypic outcomes.The continuous and consistent beating of a heart is an amazing thing. Cells in the wall of the heart, in the sinoatrial node, spontaneously produce the electrical impulse that keeps the heart muscle moving. This current, in turn, depends on specialized hyperpolarization-activated cyclic nucleotide-gated (HCN) 2 ion channels that create ion influxes, making cells easier to activate (1). The importance of the HCN channels to this process is reflected by the discovery that the S672R mutation in the cAMP-binding domain (CBD; also known as the cyclic nucleotide-binding domain of HCN4, the most abundant channel isoform), is a direct cause of familial lower heart rate or arrhythmia disorder (2). Phenotypically, the mutation causes a negative shift in the HCN channel-activation voltage, making the channel more difficult to activate, as well as accelerating its deactivation (3). However, the mechanistic details underlying this phenotype have been less clear. A new NMR analysis of the S672R mutation from Boulton et al. (4) sheds light on these aspects, pointing to HCN channel dynamics as the critical factor in modulating the beat rate.HCN channels integrate multiple signals to control ion flux across the membrane. Channel opening is primarily regulated by transmembrane elements that sense membrane hyperpolarization (5). However, the voltage for HCN activation is also regulated by tetramerization, mediated by an intracellular domain called the C-linker, which reduces the required voltage (6). Tetramerization can occur only when cAMP is bound, as this event induces a conformational change (Fig. 1A) in the other intracellular domain, the CBD, that would otherwise clash with the tetrameric C-linker conformation; in the apo form, then, auto-inhibition is observed (6). Surprisingly, the crystal structure of cAMP bound to an hHCN4 C-linker/CBD construct carrying the S672R mutation, determined by Xu et al. (7), revealed no substantial global conformational changes as compared with the wild-type structure except for a disordered loop on the cAMP-entry path. Together with the experimentally observed 10-and 3-fold decreases of cAMPbinding affinity via isothermal titration calorimetry and fluorescence anisotropy methods, respectively, Xu et al. (7) concluded that the S672R mutation simply weakened the interaction between cAMP and the channel, destabilizing the bound cAMP and promoting the closed state.Boulton et al. (4) suspected that there may be more to the structural story than the static X-ray images conveyed. The authors used a suite of NMR techniques, including heteronuclear singlequantum coherence spectroscopy (NH-HSQC), a two-dimensional 1 H, 15 N-TROSY, along with subsequent chemical ...

Precision medicine review: rare driver mutations and their biophysical classification

“…Altogether, mechanistic underpinnings that decide the mutational effects—whether statistically frequent or rare, at the tail of the distribution—can involve orthosteric or allosteric effects (Cheng and Nussinov 2018 ; Cheng et al 2016 ; Collier and Ortiz 2013 ; Feher et al 2014 ; Fetics et al 2015 ; Liu and Nussinov 2008 ; Lu et al 2015 , 2016a , b ; Marcus and Mattos 2015 ; Morra et al 2009 ; Munro et al 2018 ; Nikolaev et al 2018 ; Park et al 2018 ; Risques and Kennedy 2018 ; Shen et al 2017 ; Stout and Campbell 2018 ; Tehver et al 2009 ; Tsai and Nussinov 2014 , 2017 ; Tuncbag et al 2017 ; Waters and Der 2018 ; Xu et al 2017 ; Zhan et al 2016 ). Orthosteric mutations are at binding or active sites and can directly abolish (most frequent) or strengthen (much less frequent) interactions.…”