A high affinity switch for cAMP in the HCN pacemaker channels

Heart Rate Reduction Is Associated With Reverse Left Ventricular Remodeling and Mechanism-Specific Molecular Phenotypes in Dilated Cardiomyopathy

BACKGROUND

Heart rate (HR) affects heart failure outcomes, via uncertain mechanisms that may include left ventricular remodeling. However, in human ventricular myocardium, HR change has not been associated with a particular remodeling molecular phenotype.

METHODS

Patients with nonischemic dilated cardiomyopathy (N=22) in sinus rhythm and refractory to β-blockade for both HR lowering and reverse remodeling were randomized 2:1 double-blind to the HCN4 (hyperpolarization-activated cyclic nucleotide-gated potassium channel 4) channel inhibitor ivabradine or placebo for 24 weeks treatment while maintaining target doses of β-blockers. Reverse remodeling was measured by left ventricular ejection fraction (LVEF), and myocardial gene expression by sequencing RNA extracted from endomyocardial biopsies. The primary statistical analysis was between HR change categories divided at the median, which resulted in Decreased HR (N=90) and Unchanged HR (N=8) groups.

RESULTS

Respective HRs at baseline and 24 weeks were as follows: Decreased HR, 82.9±6.8 and 69.7±8.0 beats per minute (P=0.0005) and Unchanged HR, 80.8±5.7 and 79.2±11.6 beats per minute (P=0.58). All completing Decreased HR subjects were treated with ivabradine, whereas in the Unchanged HR group, 3 received ivabradine and 5 placebo. In Decreased HR, LVEF increased from 29.4±8.8% at baseline to 44.2±9.4% at 24 weeks (P=0.0003), compared with respective values of 26.6±11.4% and 29.2±12.0% (P=0.28) in Unchanged HR. HR and LVEF changes were not different from a previously conducted β-blocker nonischemic dilated cardiomyopathy study subdivided into LVEF responders and nonresponders. However, differentially expressed genes (N=151) in the Decreased versus Unchanged HR groups were >99% nonconcordant and therefore individually unique compared with β-blocker HR/LVEF responders versus nonresponders (2 shared differentially expressed genes). Multiple unique differentially expressed genes in Decreased HR including NRG1 upregulation are considered cardioprotective or involved in cardiac development.

CONCLUSIONS

In patients with nonischemic dilated cardiomyopathy in sinus rhythm, HR lowering per se (1) is associated with substantial left ventricular reverse remodeling; (2) its absence can cause β-blocker reverse remodeling nonresponse; and (3) when from HCN4 channel inhibition, results in a unique molecular phenotype.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique identifier: NCT02973594.

Deep Learning-Based Ion Channel Kinetics Analysis for Automated Patch Clamp Recording

The patch clamp technique is a fundamental tool for investigating ion channel dynamics and electrophysiological properties. This study proposes the first artificial intelligence framework for characterizing multiple ion channel kinetics of whole-cell recordings. The framework integrates machine learning for anomaly detection and deep learning for multi-class classification. The anomaly detection excludes recordings that are incompatible with ion channel behavior. The multi-class classification combined a 1D convolutional neural network, bidirectional long short-term memory, and an attention mechanism to capture the spatiotemporal patterns of the recordings. The framework achieves an accuracy of 97.58% in classifying 124 test datasets into six categories based on ion channel kinetics. The utility of the novel framework is demonstrated in two applications: Alzheimer's disease drug screening and nanomatrix-induced neuronal differentiation. In drug screening, the framework illustrates the inhibitory effects of memantine on endogenous channels, and antagonistic interactions among potassium, magnesium, and calcium ion channels. For nanomatrix-induced differentiation, the classifier indicates the effects of differentiation conditions on sodium and potassium channels associated with action potentials, validating the functional properties of differentiated neurons for Parkinson's disease treatment. The proposed framework is promising for enhancing the efficiency and accuracy of ion channel kinetics analysis in electrophysiological research.

HCN4 and arrhythmias: Insights into base mutations

In the human sinoatrial node (SAN), HCN4 is the primary subtype among the four HCN (hyperpolarization activated cyclic nucleotide-gated) family subtypes. A tetramer of HCN subunits forms the ion channel conducting the hyperpolarization-activated "funny" current (If), which plays an important regulatory role in maintaining the pacemaker activity of the SAN. With the advancement of detection technologies over the past 20 years, the relationship between base mutations in the HCN4 gene encoding the HCN4 protein and arrhythmias has been continuously elucidated. The expression and kinetic changes of mutated channels were investigated in COS-7, CHO, HEK-293T cells, and Xenopus oocytes, but their functional changes were not elucidated in human myocardial cells. New genome editing methods, such as Base editor and Prime editor, use components of the CRISPR system and other enzymes to directly install single-gene mutation into cellular DNA without causing double-stranded DNA breaks, which reproduce and correct base mutations. In this review, we summarize all base mutations of the HCN4 gene, discuss the clinical characteristics and function of some base mutations, and combine base editors to explore the establishment of disease models and the potential for future gene correction.

Similar Binding Modes of cGMP Analogues Limit Selectivity in Modulating Retinal CNG Channels via the Cyclic Nucleotide-Binding Domain

In treating retinitis pigmentosa, a genetic disorder causing progressive vision loss, selective inhibition of rod cyclic nucleotide-gated (CNG) channels holds promise. Blocking the increased Ca2+-influx in rod photoreceptors through CNG channels can potentially delay disease progression and improve the quality of life for patients. To find inhibitors for rod CNG channels, we investigated the impact of 16 cGMP analogues on both rod and cone CNG channels using the patch-clamp technique. Although modifications at the C8 position of the guanine ring did not change the ligand efficacy, modifications at the N1 and N2 positions rendered cGMP largely ineffective in activating retinal CNG channels. Notably, PET-cGMP displayed selective potential, favoring rod over cone, whereas Rp-cGMPS showed greater efficiency in activating cone over rod CNG channels. Ligand docking and molecular dynamics simulations on cyclic nucleotide-binding domains showed comparable binding energies and binding modes for cGMP and its analogues in both rod and cone CNG channels (CNGA1 vs CNGA3 subunits). Computational experiments on CNGB1a vs CNGB3 subunits showed similar binding modes albeit with fewer amino acid interactions with cGMP due to an inactivated conformation of their C-helix. In addition, no clear correlation could be observed between the computational scores and the CNG channel efficacy values, suggesting additional factors beyond binding strength determining ligand selectivity and potency. This study highlights the importance of looking beyond the cyclic nucleotide-binding domain and toward the gating mechanism when searching for selective modulators. Future efforts in developing selective modulators for CNG channels should prioritize targeting alternative channel domains.