Abstract Body: Background: Cardiomyocytes located in the infarct border zone (BZ) experience sudden changes in the distribution of mechanical forces following myocardial infarction (MI). How this mechanical stress affects nuclear integrity and genomic stability in surviving cardiomyocytes is unknown.
Hypothesis: Mechanical stress in the BZ induces nuclear envelope rupture in cardiomyocytes, activating a DNA damage response (DDR) that promotes survival rather than apoptosis.
Methods: Nuclear integrity was examined in a cardiomyocyte-specific nuclear reporter mouse (CM-tdTomato-NLS) following MI. Nuclear rupture was visualized by leakage of nuclear tdTomato signal into the cytoplasm. DNA damage was assessed using γH2AX immunostaining across infarct (IZ), border (BZ), and remote (RZ) zones. Cardiomyocyte transcriptional responses were analyzed using mouse and human single-nucleus RNA-sequencing datasets and spatial transcriptomics. Apoptosis was evaluated by TUNEL staining. Quantitative comparisons between zones and time points were performed using ANOVA with post-hoc testing.
Results: BZ cardiomyocytes exhibited a graded spectrum of nuclear rupture, with increased cytoplasmic tdTomato compared to intact nuclear localization in RZ cells. The proportion of nuclei with intermediate/high cytoplasmic signal was significantly increased in the BZ versus RZ (P<0.01). γH2AX staining showed marked enrichment of DDR-positive nuclei in the BZ relative to RZ (P<0.001). Integrated mouse and human single-nucleus transcriptomic datasets identified DDR gene enrichment within Xirp2+ cardiomyocytes corresponding to the previously defined BZ2 population. Spatial transcriptomics localized these DDR-enriched cardiomyocytes to the infarct border adjacent to the fibrotic interface. Despite elevated DDR signaling, the majority of γH2AX-positive cardiomyocytes were TUNEL-negative (>85%), indicating survival.
Conclusions: Mechanical stress in the infarct border zone induces transient nuclear rupture in cardiomyocytes, triggering a DDR enriched in Xirp2+ border zone cells. Rather than leading to apoptosis, DDR activation is associated with survival, suggesting recovery of nuclear integrity and an adaptive response to mechanical injury. These findings identify nuclear rupture and DDR activation as key features of cardiomyocyte adaptation in the post-infarct border zone.