Heart disease is the leading cause of death in the developed world, and its comorbidities such as hypertension, diabetes, and heart failure are accompanied by major transcriptomic changes in the heart. During cardiac dysfunction, which leads to heart failure, there are global epigenetic alterations to chromatin that occur concomitantly with morphological changes in the heart in response to acute and chronic stress. These epigenetic alterations include the reversible methylation of lysine residues on histone proteins. Lysine methylations on histones H3K4 and H3K9 were among the first methylated lysine residues identified and have been linked to gene activation and silencing, respectively. However, much less is known regarding other methylated histone residues, including histone H4K20. Trimethylation of histone H4K20 has been shown to repress gene expression; however, this modification has never been examined in the heart. Here, we utilized immunoblotting and mass spectrometry to quantify histone H4K20 trimethylation in three models of cardiac dysfunction. Our results show that lysine methylation at this site is differentially regulated in the cardiomyocyte, leading to increased H4K20 trimethylation during acute hypertrophic stress in cell models and decreased H4K20 trimethylation during sustained ischemic injury and cardiac dysfunction in animal models. In addition, we examined publicly available data sets to analyze enzymes that regulate H4K20 methylation and identified two demethylases (KDM7B and KDM7C) and two methyltransferases (KMT5A and SMYD5) that were all differentially expressed in heart failure patients. This is the first study to examine histone H4K20 trimethylation in the heart and to determine how this post-translational modification is differentially regulated in multiple models of cardiac disease.

The histone lysine methyltransferase SMYD1 has been shown to be critical for embryonic cardiac development and in maintaining cardiomyocyte homeostasis in adult mice. Subsequently, we reported that loss of Smyd1 in the adult mouse myocardium leads to progressive cardiac hypertrophy and heart failure, which is accompanied with downregulation of mitochondrial proteins involved in oxidative phosphorylation, including Ppargc1a , and reduction of mitochondrial respiration capacity. To build upon these results and evaluate if SMYD1a can attenuate disease-induced remodeling in an animal model, we generated transgenic mice which inducibly express SMYD1a (the human ortholog) in cardiomyocytes and subjected them to permanent occlusion (PO) of the LAD. This lead to >50% reduction in infarct size and preserved cardiac function, as compared to littermate controls. Additionally, we demonstrated that under physiological conditions SMYD1a maintains metabolic homeostasis by regulating expression of Ppargc1a and its downstream targets, including components of the electron transport chain. Our molecular analysis shows that observed protection from ischemic injury results from enhanced mitochondrial respiration through Complex I and II as well as increased ATP production. This is associated with increased mitochondria cristae, and formation and stabilization of respiratory chain supercomplexes within the cristae. These changes in cristae structure occur concomitant with enhanced OPA1 expression, a major regulator of mitochondrial fusion and cristae morphology. Through this work we have established that OPA1 is a novel, functionally important downstream target of SMYD1a by which cardiomyocytes upregulate energy efficiency, protecting them from ischemic injury. These results also highlight SMYD1a as the only known epigenetic regulator of cristae morphology and provide broad implications for understanding the epigenetic mechanisms driving cardiac metabolism. Ultimately this work has identified a novel signaling pathway by which cardiomyocytes regulate energy efficiency, protecting them from ischemic injury.

SMYD1 is a lysine methyltransferase, which has been shown to methylate lysine 4 on histone H3, an established mark of gene activation. SMYD1 is only expressed in skeletal and cardiac muscle and was originally shown to play a significant role in regulating cardiac development. In the adult myocardium, using inducible, cardiomyocyte-specific Smyd1 knockout mice, loss of SMYD1 leads to massive downregulation of mitochondrial bioenergetics and overt heart failure. However, the effects of SMYD1 gain-of-function in the heart and its molecular function in the cardiomyocyte in response to ischemic stress remains unknown. Here we demonstrate that SMYD1a, the mouse ortholog of human SMYD1, positively regulates cardiac energetics and protects the heart from ischemic injury. To delineate how SMYD1a controls energy efficiency and metabolism in the cardiomyocyte, we generated a novel mouse model capable of inducible cardiomyocyte-specific SMYD1a overexpression. When subjected to ischemic injury these transgenic mice display reduced infarct size and cardiomyocyte death concomitant with enhanced mitochondrial respiratory efficiency. In addition, our molecular analysis revealed that the cardiac tissue in these animals is protected from ischemic injury through SMYD1a’s synergistic regulation of two key mitochondrial pathways. First, through its histone methyltransferase activity, SMYD1a maintains metabolic homeostasis by preserving basal expression of PGC-1α and its downstream targets including electron transport chain subunits. Second, SMYD1a regulates expression of OPA1, a key regulator of cristae morphology which drives the formation of electron transport chain supercomplexes to enhance mitochondrial respiration and ATP production. This work highlights SMYD1a as the only known epigenetic regulator of cristae morphology and identifies a novel molecular pathway by which cardiomyocytes dynamically regulate energy efficiency to protect from ischemic injury.

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Heart disease is the leading cause of death in the developed world, and its comorbidities such as hypertension, diabetes, and heart failure are accompanied by major transcriptomic changes in the heart. During cardiac dysfunction, which leads to heart failure, there are global epigenetic alterations to chromatin that occur concomitantly with morphological changes in the heart in response to acute and chronic stress. These epigenetic alterations include the reversible methylation of lysine residues on histone proteins. Lysine methylation on histone H3K4 and H3K9 were among the first methylated lysine residues identified and have been linked to gene activation and silencing, respectively. However, much less is known regarding other methylated histone residues, including histone H4K20. Trimethylation of histone H4K20 has been shown to repressive gene expression, however this mark has never been examined in the heart. Here we utilized immunoblotting and mass spectrometry to quantify histone H4K20 trimethylation in three models of cardiac dysfunction. Our results show that lysine methylation at this site is regulated in a biphasic manner leading to increased H4K20 trimethylation during acute hypertrophic stress and decreased H4K20 trimethylation during sustained ischemic injury and cardiac dysfunction. In addition, we examined publicly available datasets to analyze enzymes that regulate H4K20 methylation and identified one demethylase (KDM7C) and two methyltransferases (KMT5A and SMYD5) which were all upregulated in heart failure patients. This is the first study to examine histone H4K20 trimethylation in the heart and to determine how this post-translational modification is differentially regulated in multiple models of cardiac disease.