Abstract Body: Introduction
Neonatal mammalian hearts lose their ability to regenerate shortly after birth. Identifying the mechanisms that restore this capacity is critical for developing regenerative therapies. The Hippo pathway is a conserved signaling pathway that regulates cell proliferation, survival, and differentiation. Inhibition of Hippo kinases activates the transcriptional coactivator YAP, upregulating downstream target genes. We previously reported that cardiomyocyte (CM)-specific Hippo inhibition in adult mice reverses myocardial infarction (MI)-induced heart failure. Moreover, we also observed reduced fibrosis and increased border-zone vascularity in this context, indicating that YAP activates a reparative program that extends beyond CM renewal.

Hypothesis
We assessed the hypothesis that CM-specific Hippo inhibition promotes cardiac regeneration by non–cell-autonomously reducing fibrosis and increasing vascularity.

Methods
Regeneration was induced by deleting the Hippo pathway adaptor Salvador (Sav1) specifically in CMs (Sav1^ΔCM) 3 weeks after myocardial infarction. Magnetic resonance imaging was used to assess cardiac structure and function. Hearts were collected at 6 and 9 weeks post-infarction for transcriptomic profiling, using single-nucleus RNA sequencing and Spatial Transcriptomics (n = 4 per genotype). Immunostaining and in situ hybridization were performed to validate transcriptomic results.

Results
Sav1-deficient CMs restored oxidative phosphorylation and contractile gene expression, consistent with improved cardiac performance. Spatial analysis demonstrated reduced fibrosis in the border zone, along with a reduction in pro-fibrotic fibroblast states. Pseudotime analysis revealed a shift in fibroblasts toward a reparative state resembling that of neonatal fibroblasts. In parallel, Sav1^ΔCM hearts exhibited a significant expansion of lymphatic endothelial cells (LECs) within the ischemic region. Moreover, LEC expansion strongly correlates with ejection fraction recovery, suggesting that Sav1-deleted CMs promote lymphangiogenesis to improve cardiac function.

Conclusions
In conclusion, YAP activation in CMs induces a reparative response involving multiple cell types. By reducing fibrosis and promoting lymphangiogenesis, Sav1-deleted CMs remodel the chronic ischemic microenvironment to drive cardiac regeneration.