An epigenetic circuit linking oxidative stress and DNA hydroxymethylation in heart failure

Background

Heart failure is a major cause of morbidity and mortality worldwide, but the underlying molecular mechanisms remain not well defined. Reactive oxygen species (ROS) in heart failure (HF) alter multitudes of mitochondrial enzymes and metabolites, such as α-ketoglutarate and its oxidised form L-2-hydroxyglutarate (L-2HG). These metabolites are cofactors for ten eleven translocation (TET) enzymes that convert DNA's 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC).

Aim

We hypothesize that oxidative stress during heart failure alters mitochondrial function through epigenetic remodeling, and that reduction of oxidative stress will improve cardiac function.

Methods and results

Targeted LC-MS/MS analysis of dilated cardiomyopathy (DCM) hearts obtained from MLP−/− and WT littermate controls showed a significant increase in the oxidized metabolite L-2HG (∼30%, p=0.004), with significant reduction of multiple TCA cycle intermediates. RNA sequencing revealed a significant reduction in mRNA levels of IDH2 in human TTN related DCM and MLP−/− HF mice. The altered activity of IDH2 contributes to ROS production in these hearts and to the production of L-2HG. No alteration in the mitochondria structures was observed. HF biopsies show decreased TET activity most likely due to increased L-2HG levels. Whole genome single base pair 5mC and 5hmC deep sequencing of gDNA from explanted human DCM heart biopsies and MLP−/− mouse model revealed significantly altered global distribution of both 5mC and 5hmC in comparison to control samples. Genes involved in hypertrophy, such as Myh7 and Fhl2 were among the top genes with differential 5mC levels. Global loss of 5hmC level was observed, especially in the intronic regions of genes involved in redox hemostasis. Reducing oxidative stress in vivo in MLP−/− using a small molecule (AZ14117925) improves heart function (EF) by 13% [EF=(Treated:47,44%, Placebo: 34,54%), n=5 (males per group), p=0.0373, unpaired t-test]. Additional bioinformatic analysis revealed that reduction of ROS most likely leads to activation of TET and activation of pro-survival pathways, anti-oxidative stress response, and significantly less activation of apoptotic pathways.

Conclusion

Alterations in TCA cycle metabolites may underlie changes in DNA methylation and gene expression in end-stage human DCM and mouse models and indicate a role for epigenetic regulation of mitochondrial function in HF. NRF2 activation may pose a novel therapeutic approach to treat this devastating disease.

Funding Acknowledgement

Type of funding source: Private company. Main funding source(s): AstraZeneca

Cross-disorder risk gene CACNA1C differentially modulates susceptibility to psychiatric disorders during development and adulthood

Single-nucleotide polymorphisms (SNPs) in CACNA1C, the α1C subunit of the voltage-gated L-type calcium channel Cav1.2, rank among the most consistent and replicable genetics findings in psychiatry and have been associated with schizophrenia, bipolar disorder and major depression. However, genetic variants of complex diseases often only confer a marginal increase in disease risk, which is additionally influenced by the environment. Here we show that embryonic deletion of Cacna1c in forebrain glutamatergic neurons promotes the manifestation of endophenotypes related to psychiatric disorders including cognitive decline, impaired synaptic plasticity, reduced sociability, hyperactivity and increased anxiety. Additional analyses revealed that depletion of Cacna1c during embryonic development also increases the susceptibility to chronic stress, which suggest that Cav1.2 interacts with the environment to shape disease vulnerability. Remarkably, this was not observed when Cacna1c was deleted in glutamatergic neurons during adulthood, where the later deletion even improved cognitive flexibility, strengthened synaptic plasticity and induced stress resilience. In a parallel gene × environment design in humans, we additionally demonstrate that SNPs in CACNA1C significantly interact with adverse life events to alter the risk to develop symptoms of psychiatric disorders. Overall, our results further validate Cacna1c as a cross-disorder risk gene in mice and humans, and additionally suggest a differential role for Cav1.2 during development and adulthood in shaping cognition, sociability, emotional behavior and stress susceptibility. This may prompt the consideration for pharmacological manipulation of Cav1.2 in neuropsychiatric disorders with developmental and/or stress-related origins.

Analysis of calcium channels by conditional mutagenesis

Ca2+ influx through various ion channels is an important determinant of the cytosolic Ca2+ concentration, which plays a pivotal role in countless cellular processes. The cardiac L-type Ca2+ channel, Ca(v)1.2, represents a major pathway for Ca2+ entry and is in many cells expressed together with other high- and low-voltage-activated Ca2+ channels. This article will focus on the use of conditional transgenic mouse models to clarify the roles of Ca2+ channels in several biological systems. The phenotypes of conditional Ca2+ channel transgenic mice have provided novel, and often unexpected, insights into the in vivo function of L-type and T-type Ca2+ channels as mediators of signaling between cell membrane and intracellular processes in blood pressure regulation, smooth muscle contractility, insulin secretion, cardiac function, sleep, learning, and memory.

Cellular expression and functional characterization of four hyperpolarization-activated pacemaker channels in cardiac and neuronal tissues

Hyperpolarization-activated cation currents (I(h)) have been identified in cardiac pacemaker cells and a variety of central and peripheral neurons. Four members of a gene family encoding hyperpolarization-activated, cyclic nucleotide-gated cation channels (HCN1--4) have been cloned recently. Native I(h) currents recorded from different cell types exhibit distinct activation kinetics. To determine if this diversity of I(h) currents may be caused by differential expression of HCN channel isoforms, we investigated the cellular distribution of the transcripts of HCN1--4 in the murine sinoatrial node, retina and dorsal root ganglion (DRG) by in situ hybridization. In the sinoatrial node, the most prominently expressed HCN channel is HCN4, whereas HCN2 and HCN1 are detected there at moderate and low levels, respectively. Retinal photoreceptors express high levels of HCN1, whereas HCN2, 3 and 4 were not found in these cells. In DRG neurons, the dominant HCN transcript is HCN1, followed by HCN2. We next determined the functional properties of recombinant HCN1--4 channels expressed in HEK293 cells. All four channel types gave rise to I(h) currents but displayed marked differences in their activation kinetics. Our results suggest that the heterogeneity of native I(h) currents is generated, at least in part, by the tissue-specific expression of HCN channel genes.

Expression of T- and L-type calcium channel mRNA in murine sinoatrial node

At the cellular level, cardiac pacemaking which sets the rate and rhythm of the heartbeat is produced by the slow diastolic depolarization. Several ion channels contribute to this pacemaker depolarization, including T-type and L-type calcium currents. To evaluate the molecular basis of the currents involved, we investigated the cellular distribution of various low voltage activated (LVA) and high voltage activated (HVA) calcium channel mRNAs in the murine sinoatrial (SA) node by in-situ hybridization. The most prominently expressed LVA calcium channel in the SA node is Ca(v)3.1, whereas Ca(v)3.2 is present at moderate levels. The dominant HVA calcium channel transcript is Ca(v)1.2; only traces of Ca(v)1.3 mRNA are detectable in SA myocytes of mice.

Differential distribution of four hyperpolarization-activated cation channels in mouse brain

Hyperpolarization-activated cation currents, termed I(h), are observed in a variety of neurons. Four members of a gene family encoding hyperpolarization-activated cyclic-nucleotide-gated cation channels (HCN1-4) have been cloned. The regional expression and cellular localization of the four HCN channel types in mouse brain was investigated using in situ hybridization. The expression of HCN1 was restricted to the olfactory bulb, cerebral cortex, hippocampus, superior colliculus and cerebellum. In contrast, HCN2 transcripts were found at high levels nearly ubiquitously in the brain, and the strongest signals were seen in the olfactory bulb, hippocampus, thalamus and brain stem. HCN3 was uniformly expressed at very low levels throughout the brain. Finally, HCN4 transcripts were prominently expressed selectively in the thalamus and olfactory bulb. Some neurons expressed two or more HCN channel transcripts including hippocampal pyramidal neurons (HCN1, HCN2 and low levels of HCN 4) and thalamic relay neurons (HCN2 and HCN4). Our results demonstrate that each HCN channel transcript has a unique distribution in the brain. Furthermore, they suggest that the heterogeneity of neuronal I(h) may be, at least in part, due to the differential expression of HCN channel genes.