Cardiomyocyte-specific loss of Smyd5 leads to a robust activation of inflammatory signaling and heart failure in mice

Abstract

Background Cardiomyocytes respond to stress by undergoing hypertrophic growth driven by dynamic changes in gene expression. Epigenetic mechanisms, including histone methylation, play critical roles in regulating these transcriptional programs, yet the enzymes controlling these modifications during cardiac disease remain largely unknown. The SMYD family of histone methyltransferases regulates gene expression in multiple biological contexts, but the function of SMYD5 in the mammalian heart has never been investigated. Methods SMYD5 expression was assessed in human heart failure samples and in a mouse model of cardiac hypertrophy. To define its functional role in vivo, we generated inducible cardiomyocyte-specific Smyd5 knockout mice and characterized their cardiac phenotype using molecular, histological, and functional analyses. Chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) was performed to examine histone H4 lysine 20 trimethylation (H4K20me3) at the Il-6 promoter. Results SMYD5 expression was altered in diseased human and mouse hearts. Under basal conditions, cardiomyocyte-specific deletion of Smyd5 resulted in baseline structural cardiac remodeling and transcriptional signatures characteristic of pathological stress. Smyd5-deficient hearts exhibited marked inflammatory activation resembling a cytokine storm with immune cell infiltration and heart failure. Notably, Smyd5 knockout mice displayed a 100-fold increase in Il-6 expression, accompanied by a global reduction in H4K20me3. ChIP-qPCR analysis of the Il-6 promoter, together with loss- and gain-of-function analysis of SMYD5, supports a direct epigenetic role of SMYD5 in regulating Il-6 expression through H4K20me3 in cardiomyocytes. Conclusions SMYD5 is a previously unrecognized epigenetic regulator of cardiac homeostasis that restrains inflammatory signaling in cardiomyocytes under normal conditions. Loss of Smyd5 disrupts H4K20me3, leading to derepression of Il-6 in cardiomyocytes and a robust inflammatory response characterized by immune cell recruitment and fibrosis, accompanied by rapid progression of cardiac remodeling and heart failure. These findings identify SMYD5 as a critical regulator of intrinsic cardiomyocyte inflammatory signaling and reveal a novel chromatin-based mechanism contributing to inflammatory cardiomyopathies. NOVELTY AND SIGNIFICANCE What Is Known? Elevated levels of pro-inflammatory cytokines such as IL-6 are strongly associated with adverse cardiac remodeling and poor outcomes in heart failure. Histone methylation is a key epigenetic mechanism regulating gene expression in the heart, but the role of histone H4K20 trimethylation and its regulatory enzymes in cardiomyocytes remains poorly understood. The histone methyltransferase SMYD5 regulates gene expression and inflammatory pathways in several non-cardiac cell types, but its role in the mammalian heart has not been examined. What New Information Does This Article Contribute? This study provides the first in vivo characterization of SMYD5 in the heart and identifies it as a critical epigenetic regulator of cardiomyocyte homeostasis. Cardiomyocyte-specific loss of Smyd5 triggers pathological hypertrophy, elevated inflammatory signaling, immune cell infiltration, fibrosis, and heart failure. SMYD5 directly represses Il-6 expression in cardiomyocytes through histone H4K20 trimethylation at the Il-6 promoter, revealing a previously unrecognized epigenetic mechanism controlling intrinsic cardiomyocyte-driven inflammation. Epigenetic mechanisms that regulate inflammatory signaling in cardiomyocytes remain largely unknown. Here, we identify the histone methyltransferase SMYD5 as a critical regulator of cardiac homeostasis and intrinsic inflammatory signaling. Using an inducible cardiomyocyte-specific Smyd5 knockout mouse model, we demonstrate that loss of Smyd5 induces rapid progression to heart failure accompanied by robust inflammatory activation, including a ∼100-fold increase in Il-6 expression, inflammatory immune cell infiltration, fibrosis, and severe cardiac dysfunction. Mechanistically, SMYD5 directly regulates Il-6 expression by catalyzing histone H4 lysine 20 trimethylation at the Il-6 promoter, thereby restraining pro-inflammatory gene expression in cardiomyocytes. Ablation of Smyd5 markedly reduces global H4K20 trimethylation, and results in dramatic upregulation of Il-6 and downstream cytokine signaling pathways, producing a phenotype resembling cytokine storm-like inflammatory cardiomyopathy. These findings establish SMYD5 as the first epigenetic regulator shown to suppress intrinsic cardiomyocyte inflammatory signaling and uncover a novel chromatin-based mechanism controlling cytokine production in the heart. Targeting SMYD5-dependent pathways therefore represents a new strategy for limiting maladaptive inflammation in heart failure and inflammatory cardiomyopathies.