IF cardiac-targeted lipid nanoparticles (CT-LNPs) co-encapsulating (1) NRG1 mRNA and (2) mRNA encoding the H3K9me3-specific histone demethylase KDM4D, formulated with an ionizable lipid core and surface-conjugated with a cardiac-homing peptide (e.g., CRPPR or equivalent cardiomyocyte-targeting ligand), are administered via intramyocardial or intravenous injection at a dose of 0.5–1.0 mg/kg total mRNA payload per injection, in a 3-dose regimen over 7 days, to aged male and female C57BL/6J mice (22–24 months), four weeks after non-reperfused myocardial infarction (MI),

THEN a significant increase in cardiomyocyte cell-cycle re-entry (≥3-fold increase in Ki67⁺/troponin T⁺ co-positive nuclei; ≥2-fold increase in pH3⁺ and Aurora B midzone-positive cardiomyocytes at the cytokinesis furrow) will be observed relative to aged MI controls receiving either NRG1 mRNA-only LNPs or vehicle, with concomitant improvement in ejection fraction (≥8–10 percentage point gain by echocardiography at 8 weeks post-MI), reduction in infarct scar area (Masson's trichrome), and reduced H3K9me3 immunofluorescence intensity at the CCND1 and CDKN1A loci in cardiomyocyte nuclei (assessed by CUT&RUN),

BECAUSE the following mechanistic chain is operative:

1. Epigenetic lockdown is the proximal barrier in aged hearts, not ligand insufficiency. The evidence set demonstrates that aged mouse cardiomyocytes accumulate dense H3K9me3 and H3K27me3 repressive histone marks at the promoters of pro-proliferative and fetal gene programs, physically barring transcription factor access even when upstream receptor signaling (ErbB2/ErbB4) is intact. Consequently, exogenous NRG1 protein or mRNA delivered alone to aged hearts fails to translate receptor activation into meaningful cell-cycle progression. (Epigenetic silencing at cardiomyocyte proliferative loci in aging)[Annual Review of Pathology; Nature Reviews Cardiology as cited in Evidence Set]

2. KDM4D delivery reverses H3K9me3-mediated heterochromatin at proliferative gene loci, restoring chromatin accessibility. KDM4D is a member of the KDM4 Jumonji-domain demethylase family with documented selectivity for tri- and di-methylated H3K9. Transient overexpression of KDM4 family members via mRNA delivery would be self-limiting (no genomic integration; protein decays within 24–72 hours), reducing the risk of sustained off-target demethylation. By re-opening chromatin at the CCND1, CDK4, and E2F target gene promoters while simultaneously reducing epigenetic repression at CDKN1A (p21) loci, KDM4D mRNA expression creates a permissive chromatin window for mitogenic signal transduction. (H3K9me3 accumulation at cardiomyocyte proliferation loci with age)[Nature Reviews Cardiology as cited in Evidence Set] [SPECULATIVE: that KDM4D selectivity for proliferative versus tumor-suppressor loci is sufficient to avoid global heterochromatin collapse — requires CUT&RUN validation]

3. **NRG1 mRNA, translated within the same cardiomyocyte following L...

SENS category: RepleniSENS