Abstract Body: Physiological differences between the ventricular endomyocardium (ENDO) and epimyocardium (EPI), termed transmural gradients, are essential for coordinated cardiac pumping. Although their disruption is associated with arrhythmias and heart failure, their transcriptional basis and remodeling in disease remain poorly defined. Here, we define genome-wide ventricular transmural gradients and examine the role of the transcription factor Irx5 in regulating them in physiological and pressure overload-induced hypertrophic hearts.

Bulk RNA-sequencing analysis of left ventricular ENDO and EPI tissues identified over 1,000 differentially expressed genes in non-failing adult human hearts and hundreds in adult mouse hearts. Cross-species comparison revealed conserved region-specific functional enrichment, such as sarcomere contraction in ENDO, and fatty acid metabolism in EPI. Notably, IRX5 was the top differentially expressed transcription factor in both species. In addition, more than half of genes with conserved transmural differences lost their gradient in Irx5 knockout (KO) mice, suggesting Irx5 as a conserved regulator of transmural gene expression. We then examined transmural gradients and their dependence on Irx5 in the diseased heart using wild-type (WT) and Irx5 KO mice subjected to SHAM or transverse aortic constriction (TAC) surgery for 2 weeks. WT hearts developed significant cardiomyocyte hypertrophy and interstitial fibrosis predominantly in ENDO, whereas these changes were largely reduced in Irx5 KO hearts. Transcriptomic profiling revealed hundreds of gradients lost or newly formed after TAC, indicating extensive transmural gradient remodeling during hypertrophy. For example, ENDO preferentially lost gradients related to contractility and gained gradients related to lipid metabolism, while EPI lost gradients related to developmental patterning and gained gradients related to contractility. Importantly, loss of Irx5 altered more than one-third of these stress-induced changes by restoring gradients lost with TAC or abolishing gradients induced by TAC.

Altogether, our data establish Irx5 as a crucial regulator of transmural transcriptomic heterogeneities in the ventricular wall of the normal and diseased heart.