DDX21-mediated epigenetic remodeling drives cardiac fibroblast activation and fibrosis post-myocardial infarction by stabilizing SLC16A3 expression

Background Pathological cardiac fibrosis following myocardial infarction (MI) is primarily driven by the activation and differentiation of cardiac fibroblasts (CFs) into myofibroblasts—a process critically dependent on enhanced aerobic glycolysis. Nevertheless, the epigenetic mechanisms that couple metabolic reprogramming with pro-fibrotic gene expression in CFs remain largely elusive. Objective This study sought to delineate the role of the RNA helicase DDX21 in post-MI fibrosis and to uncover its non-canonical function in orchestrating histone methylation, lactate metabolism, and CF activation. Methods DDX21 expression was assessed in activated CFs isolated from murine post-MI hearts and in TGF-β-stimulated human CFs using qRT-PCR and immunoblotting. To dissect the underlying molecular mechanisms, we employed an integrative approach combining RNA-sequencing, CUT&Tag assay, and co-immunoprecipitation coupled with mass spectrometry. The functional relevance of DDX21 was further interrogated in vivo through cardiac-specific knockdown of DDX21 via adeno-associated virus (AAV) delivery in a mouse model of MI, followed by comprehensive evaluation of cardiac function, fibrotic remodeling, and molecular alterations. Results DDX21 expression was markedly upregulated in activated CFs from both murine infarcted hearts and TGF-β-treated human CFs. Mechanistically, nuclear DDX21 leveraged its protein-interaction domain to competitively engage the PHD1 domain of the histone demethylase KDM5A, thereby displacing histone H3 binding. This competitive interaction attenuated KDM5A enzymatic activity, resulting in global enrichment of H3K4me3. Notably, this epigenetic shift led to promoter hypermethylation and increased chromatin accessibility at the SLC16A3 locus, which encodes the lactate transporter MCT4. The consequent upregulation of MCT4 facilitated lactate efflux from CFs, sustaining the elevated glycolytic flux required for persistent myofibroblast activation and collagen deposition. Conversely, AAV-mediated cardiac-specific silencing of DDX21 in post-MI mice disrupted the DDX21-KDM5A interaction, reinstated KDM5A demethylase activity, diminished H3K4me3 occupancy at the SLC16A3 promoter, normalized MCT4 expression, and suppressed glycolytic activity in CFs. These interventions markedly attenuated myocardial fibrosis, preserved left ventricular ejection fraction, and reduced infarct size. Conclusion We identify a novel DDX21-KDM5A regulatory axis linking epigenetic control to metabolic reprogramming in fibrosis. By maintaining SLC16A3 promoter methylation, DDX21 drives MCT4-mediated lactate export, fueling CF glycolytic capacity and differentiation. Targeting DDX21 or its interaction with KDM5A represents a promising therapeutic strategy for heart failure.Schematic diagram