Abstract Body (Do not enter title and authors here): Heart transplantation remains a vital treatment for end-stage heart failure, yet the scarcity of suitable donor hearts makes the treatment unavailable to many patients in critical need. Aged donor hearts, despite their availability, exhibit increased primary graft dysfunction and decreased survival rates post-transplantation. Addressing this critical barrier could potentially improve post-transplant outcomes and make older donor hearts a viable option. In this study, using a mouse heterotopic heart transplant model, we first confirmed that hypothermic cardioprotection was significantly impaired in aged donor hearts but preserved in young donor hearts. RNA-Seq analyses revealed down-regulation of RNA editing activity along with exacerbated Endoplasmic reticulum stress (ER stress) and increased unfolded protein response (UPR) activity in aged donor hearts post-transplantation. Supplementing cardioplegic solution with rapamycin, an inhibitor of both ER stress and UPR, relieved graft heart dysfunction in aged mice, indicating that ER stress and UPR are responsible for this dysfunction.

Furthermore, we found that Adenosine Deaminase Acting on RNA-1 (ADAR1) was downregulated in aged donor hearts post-transplantation in both our mouse model and in human patients. Importantly, ADAR1 deficiency in young donor hearts abolished hypothermic cardioprotection, leading to increased UPR activity, ER stress, and cellular senescence. Conversely, overexpression of ADAR1 in aged donor hearts by a novel PEG-based nanoparticle delivery in cardioplegic solution enhanced hypothermic cardioprotection by decreasing UPR activity, ER stress, and cellular senescence.

Mechanistically, ADAR1 controls the expression of p16. In aged donor hearts, ADAR1 downregulation caused an increased expression of p16, which competed for its binding with CDK4/6, leading to a decreased phosphorylation of Rb and subsequent cellular senescence.

These findings suggest that ADAR1-associated RNA editing activities play a crucial role in the hypothermic cardioprotection of donor hearts. Supplementing cardioplegic solutions with nanoparticles that enhance ADAR1 activity or modulate ER stress responses may provide a promising therapeutic strategy to improve the viability and/or function of aged donor hearts in transplantation.